Your wife wants easy attachment use and low profile!
I think a canister vacuum or central vacuum system is the right system for you but my recommendation is a canister with electric power nozzle.
From your original post your wife is the primary user and she doesn't like how hard it is to maneuver the bulky Dyson. She also wants easy attachment conversion and low profile head for getting under things. You also mentioned about cord length and having to drag a heavy unit around the house.
Carpet manufacturers for true actual Berber do not recommend a rotating brush head for Berber. That said they also recommend no pets on it, no high heel shoes and a lot of other things that the majority of practical households in the United States have no clue about and get along with their Berber just great!! Until reading about Berber today after reading some posts above, I would've had no clue about not using a powered brush roll on Berber. I've tried straight suction on it before and with regular household dirt like sand and hair and dust it didn't seem to pull it from the over lapped rows of Berber. I've always vacuumed berber carpeting with spinning brush roll and have never had a problem and find that with the right machine it comes out gorgeous!! FYI: I've done professional house cleaning for over 15 years and have had a few clients with Berber. Never ever ever had a problem vacuuming or shampooing it with a decent vacuum cleaner!!
For your situation with what you've posted about how your wife cleans I HIGHLY recommend avoiding a Kirby avoiding Dyson, avoiding Oreck. The Oreck style would be lightweight but has no tools and no brush roll on/off. The Kirby would clean very well but requires a fair amount of effort to change to tools which can't store on board. You'd be going back and forth to your closet for attachments when you need them. Kirbys are reliable, powerful cleaners but the fact is many average American consumers get sick of them quickly due to heft, bulk and inconvenience. I'm sure your local vacuum dealership has several refurbished Kirbys sitting around that no ones buying! Some people, especially vacuum collectors love them for their shiny metal and lots of attachment options that they get sold into through very high pressure sales but its the other factors that turn everyday consumers away and I recommend you do the same! I've been to many households where they bought a Kirby and it sits around in a closet or garage collecting dust while they use a lighterweight Kenmore or similar similar because the Kirby is too much of a hassle!! And the Self Propel feature on any vacuum including the Hoover Windtunnel Self Propelled isn't the easiest feature to work with either.
You should visit your local vacuum dealership and check out the Riccar Prima, Simplicity Wonder or other power nozzle canister equipped Riccar/ Simplicity canister vacuums. These canisters are U.S.A made, have attachments stored on wands with some models having additional tools on canister unit themselves. The Riccar Prima line has a long 24' cord, plus with canisters you get an additional 10' average of reach between hose, wands and unit. These canister vacuums are very very powerful with variable speed suction motors, several carpet height adjustment settings, brush roll on/off switch. Riccar themselves or your local vacuum store offer a variety of low priced straight suction tools for you to attach to your Riccar, Simplicity or other branded canister for your Berber if you preferred not to use the power nozzle with brushes turned on or off.
Don't worry about MSRP's on Riccar website either they're much lower in stores and are worth it cause of their performance, filtration options and reliability. If you're more budgeted though you might look to a Panasonic canister, or refurbed machine. Aerus canisters are like your older Electrolux Epic and do perform well. I like them but they're not my favorite due to only fitting Electrolux tools, the upholstery nozzles have no bristle or other grooming device and many times I find they flip on sides and skid around house rather than roll on wheels.
Central vacuums are powerful systems. There are many issues though. As another user stated the air flow is less powerful the further from your unit. In every house I've ever cleaned with a central vacuum, there is always some back part of a room that the hose is not able to reach due to setup and installation of the wall connections for the hose. I guess installers get cheap and don't measure square footage or around furniture and doorways well. But I've always had to grab a regular vacuum in the midst of working with central vacuum in-order to finish a job cause their central can't reach!! A corded vac will NEVER leave you not be able to reach a certain part of your home. The hoses and tools are very cumbersome! Generally with central vacs, there is no on wand attachment storage. You'll be having back and forth trips to closed to grab your accessories while tripping and struggling with a long hose. I've tripped myself on stairs before trying to travel with hose wrapped up and power nozzle wand setup in another hand! I've never tripped myself with a canister vacuum before and I've used probably over 50 different models in homes!!
I hope this information is helpful to you. As I said I've cleaned houses professionally for over 15 years. I'm very meticulous about cleaning and fussy about equipment that cleans well but is yet user friendly and durably built to fit a budget. I've helped several of my clients shop vacuums and also sold vacuums Door to Door before in sales. I noticed someone recommended a Filter Queen to you. After selling the most current market model just recently while working for Filter Queen and listening to your situation I would definitely avoid that system. You can do better and much cheaper and much easier to maintain with a Riccar or Simplicity!
I do not work for or have any company connection to Riccar or Simplicity. I receive nothing for recommending their products. I am just very satisfied by their current market equipment and that their products are USA made as well. I prefer their features, and performance over a Sebo, Aerus, Filter Queen, Dyson and a few other others. I'd be more than happy to offer more advice to you about various machines you may be looking at and you can feel free to email me through my profile!
http://riccar.com/prima-power-team-canister