Kind of an apples to oranges comparison, in most cases. In addition to KR's comments, I would point out that the prepro section of a Denon 3311 is not in same league as, say, an Anthem prepro. Nevertheless, there have been examples that defy your maxim in the past. For example, I own a Sherwood-Newcastle P-965 pre-pro. When last available, it retailed for $1500 (street price lower). The receiver version of this prepro was priced at $2000, and was identical save the addition of a 7-channel amplifer. There are a few low cost alternatives that seem to offer full, modern feature sets at low prices. These are mostly from direct mail sellers, such as Outlaw Audio and Emotive Audio. The advantage to buying direct is that these sellers usually offer a return window, so you can audition the preopro in your room, with your gear, and see if it's for you or not. Although the Outlaw is OEM'd from Sherwood, I haven't heard the current model, so I can't speak directly as to its sound quality, but, for the money, I've always found the Sherwood prepro to be very musical, if a bit short on soundstage dimensionality and transparency.
Depending on how important stereo music is to you, vs. surround sound for films, you might consider aiming for something like what I have, which combines a 2-channel system for music with a surround system for films. I am not too fusy about soundtrack reproduction, so I have focused more time and money on the 2-channel chain than the surround chain.
One would hope that a dedicated prepro would out-perform the prepro section of a budget-friendly AVR, but, as KR says, there are economies of scale to consider that benefit some manufacturers more than others. In the case of Sherwood, which OEMs for many other mid-fi companies, there is a benefit. Others, not so much. But Kal is right, you gotta listen. I was rather surprised that my Sherwood was better sonically than the then current entry-level prepro from Rotel. The Rotel was good, but in my home, in direct comparison, the Sherwood, which has a bigger feature set, sounded significantly better, IMHO.
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