Deeper and deeper...
Only a carpet with pile deeper than 3/4 of an inch cannot be adequately cleaned by an Oreck XL21, or Pilot. Not sure about the Oreck Edge, because it is the most powerful upright Oreck has ever made, but quite new to the market, and Oreck does not seem to be pushing it.
An older Hoover, a Sanitaire, a Royal metal upright, or a Kirby is capable of lifting the carpet and giving it a thorough shaking. Vibrating a carpet that is flat on the floor will removed deeply embedded sand and dirt also, but only with very powerful suction and airflow behind the agitation. Lifting the carpet makes the sweeping action more effective, and lesser suction can pull the dirt into the bag because of the higher volume of airflow.
No matter how effective a power nozzle canister, or a clean fan upright may be, it is due largely to tweaking the design and boosting the motor power.
Before we went nuts over filtering every last particle our vacuums inhale, a 3 amp motor with positive agitation, a good belt and brush roll was sufficient to dig the deepest dirt out of carpets with pile 1" deep or less.
Clean air/direct air is one major distinction in vacuum design. In my view we need to acknowledge another distinction when talking about carpet cleaning: positive agitation wherein the carpet is picked up and held by the suction of the machine while it is vibrated, or passive agitation, where the carpet is agitated while lying flat on the floor and NOT picked up by the suction of the vacuum cleaner.
To my knowledge, no one has ever invented an effective quick changeover to attachment use for an upright employing positive agitation, actually sucking up and holding the carpet off the floor while it is being cleaned.
So whether or not an Oreck, or any other vacuum cleaner deep cleans is dependent on a number of factors:
Is the carpet glued down? How dirty is it? Is the vacuum at peak efficiency? How thick is the carpet? Does it really need to be cleaned with a positive agitation upright to get all the dirt?
You can test all you want, but a positive agitation upright with a cloth bag will remove more heavy sand and dirt more quickly than any other arrangement. It is the simplest, most energy efficient design.