Here's my explanation for a lot of your questions.
***I am sorry that this got so long. I have put some headings to split it up a little!
Filtration
Many sources tend to still think of "Direct Air", also known as "Dirty Air", as the old fashioned vacuum cleaner technology. Likewise many older vacuums that had this technology simply had a paper bag and then the air was exhausted. HEPA was not a thought at this time!!
A direct air machine actually has two areas on the machine where air will be exhausted. The first area of exhausted air is where the bag is. Only a low percentage of direct air units have a hard bag cover; most are soft cloth outer bag. For these you need to shop well and research that a HEPA bag is available, if that is the filtration level you're seeking. Generally the outer bags themselves are not HEPA rated. It is important to know for this that there are varying grades of what can be rated HEPA as well. True HEPA is 99.97% of all particulate down to .3 micron. Unfortunately many manufacturers won't tell you the exact specs of their HEPA grade!! But passing filthy air through a single thin layer of so-called HEPA is not as good as going through a second thick filter.
The second area of exhaust on a Direct Air unit is at the motor area. This is air that has been specifically brought into the unit to cool the motor and is then exhausted by the motor. Unfortunately there is generally NO filter for any of this area. Due to many systems not being fully sealed, there also tends to be a fair volume of dust in this air that escaped the vacuum path somehow and ended up in the motor area. This air also has emissions from the carbon brushes that the motor uses to operate. It is for these reasons above that Direct Air units are considered to not have as good of filtration as Bypass systems.
A bypass system generally has a single exhaust. Instead of a separate entrance needed for incoming air to come in and cool the motor and then be exhausted, this system instead uses the air that the dirt traveled in, which then passed through a combination of filters to then cool the motor and then be passed through another filter. So in a good Bypass system the air for cleaning has gone through a bag or cylcone, a premotor filter and then a final HEPA filter. Therefore there are little emissions from the motors carbon brushes being released or otherwise. The HEPA filters on these units are also thick as a HEPA is supposed to be so it is a true HEPA system. Also in-case a hole was to form in the bag that air is still filtered again!
Brush rolls
Brush roll, agitator, some Eureka models called it a Disturbulator. Some people call it a beater bar!!-- That's where it gets confusing. The brush roll should not be referred to a beater bar, though it can contain beater bars. Beater bars were plastic or metal bars put on the vacuums brush roll to increase the agitation. The point of a brush roll is to not only use bristles to sweep up pet hair and lint and groom a surface but also to create an Earthquake type of effect on the carpet to vibrate dirt from the bottom of the pile to the top where the bristles and air flow work in unison to carry the dirt away. However, beater bars have been replaced by a 2nd row of bristles in over 95% of the markets machines. With good bristle design and layout, outstanding agitation is achieved. Generally a chevron design is highly recommended to guide the dirt towards the air duct.
Some brands like Oreck market that their brush roll is good on hard flooring. But do you really want to risk a high speed powered brush roll on your delicate floors??? Do you want to see similar brush roll grooming marks on your hard flooring that you'd see on carpet??? I hope you answered NO!! The best tool for cleaning bare floors is a horsehair bristle equipped bare floor brush which is attached to the suction wand of a hose on a vacuum. Miele and Riccar or Simplicity have the best designed ones for this as well. A good bare floor tool can elongate how much time you need to go between wet cleaning of your hard floors. Kitchens and bathrooms are a different story, but other rooms such as dining room, living room, bedrooms, the use a good bare floor tool will make your life much easier and will not damage your flooring during the cleaning process!! I would try to avoid any bare floor brush that has wheels on it!!
AirFlow/ Suction/ Carpet Height
As far as carpet height, a Kirby explanation is the best. Set your vacuum to the highest setting it has. Turn the vacuum on and push it. Gradually lower the vacuums cleaning head one setting at a time until the tone of the machine changes and you start seeing grooming marks. Go one setting lower than that and you should be good for height setting.
The suction seals the rug to the vacuum by pulling the carpet up to the vacuum. This action isn't highly noticeable until reaching the edge of an area rug, door mat etc, but trust and believe a good vacuum is doing it the entire time it cleans. The exception is on some carpets that are glued down. But suction doesn't remove the dirt. This is where air flow comes in. If the seal to a surface is too strong then air can not enter and therefore the dirt won't go away. The suction pulls in air and the air carries the dirt with it as it travels through the vacuum system. You may notice upon viewing the bottom plate that surrounds the brush roll area of a vacuum cleaner that it generally is not the same flatness all around. Usually along the front and side are some grooves. These grooves allow air to come into the path. The grooves also help to reduce a snow plow effect and can help the machine better gobble up chunky items, such as Cheerios.
Manufacturer
Tacony is a U.S vacuum manufacturer producing vacuum cleaners under the brands of Riccar, Simplicity, Fuller Brush, Maytag, Carpet-Pro, the Aerus Lite and other vacuums. One of their Maytag uprights is shown in the Youtube video below and you can see how good agitation using a brush roll with only bristles effectively cleans and has no plastic beater bars!! In terms of vacuum cleaners Tacony products are fantastic and highly rated!! You won't go wrong with a Riccar, Simplicity, Fuller Brush or Maytag vacuum.
Kirbys clean well. But brand new they are around $1700. They are heavy, can be awkward, the only way to shut off the brush is to turn off the unit or remove the brush roll assembly altogether. Attachments are in a separate caddy that would have to be carried separate and it's not the quickest process to exchange. I used to do in home product demonstrations for both Rainbow and Filter Queen cleaning systems. I came across MANY Kirby owners. Sadly only a small handful of those owners liked the units. The rest felt that they were highly pressured into buying a very expensive unit that was too heavy and a lot of effort to use. They had purchased a Kenmore, Hoover, Riccar, or Bissell as an easier to use vacuum and rarely used their Kirby.
Dysons represent the best design for how a bagless system works. In general on most of their units the cyclonic action works well at separately the dirt from the air and filter life lasting longer. Suction and airflow through the hose are generally very good. However Dysons have not been tested well at providing good agitation and they typically don't have a good outlet of getting sufficient airflow to the carpet cleaning head for a thorough clean. Additionally some of their brush roll designs have had bristle tufts that felt like pieces of concrete. Some carpet manufacturers, such as Mohawk actually void your warranty if a Dyson is used due to agitator damage and insufficient cleaning.
No matter your budget Tacony has a product line for you!! Fuller Brush is on the budget friendly end but a great cleaner. For power nozzle canisters the Simplicity Verve or new Wonder and Prima lines are fantastic!!! Miele is also highly rated and Aerus is a great brand.
Central Vacuum Thought
Also have you thought about a central vacuum??? That might be of interest to you!! Numerous options are available and the air is exhausted outside. There are Hide-A-Hose systems that store the hose in the wall. Wally Flex systems for a hand held hose great at cleaning up in the garage or laundry area, Vroom hose system that can store underneath your kitchen sink to clean up your kitchen floor, power nozzle kits. Central vacuums have come a long way with various additions.
http://https//www.youtube.com/watch?v=pVfQ_SA1tHY
