I've heard your concern expressed before, and the only thing I can say is, it isn't true in my experience. I now use an older B&K AVR 507 receiver and a separate 2-channel amp, and did the same in the past with an older and much more modest Denon AVR.
I haven't heard a system with the Marantz 8801/02 preamp/processors, but I have heard other very good systems with dedicated 2-channel preamps or multi-channel preamp/processors. I really don't think paying extra for them delivers an audible increase in sound quality.
In the case of B&K, a now defunct high-priced and high-quality American receiver, preamp, & amp maker, they readily admitted their AV receivers contained identical preamp sections as their more expensive multi-channel preamp/processor did. It didn't make financial sense to purchase & stock all those extra parts. I don't know what Marantz does, but I wonder if the same doesn't apply to them. Certainly the same economy of scale pressures will apply to them. It is possible that the Marantz AVRs and preamp/processors use many of the same parts, and that Marantz charges more for the preamp/processors only because the audio market for them is smaller and seems to tolerate the higher price.