Very true IMO. The HD800 sounds distant and uninvolving with a setup that's not matched for them (even if it's relatively high-end). The HD650, OTOH, sounds good from a much wider range of equipment. In fact, I prefer the HD650s from low to mid-fi gear. The HD800 has significantly more potential and technical ability, so with a good setup built around them, they do clearly overtake the 650s. But as expected, that requires a lot of time, money and effort to produce.
I definitely agree with your first bolded sentence but disagree with part of the latter. There are modestly priced amps out there that get the job done in my experience. They are just few and far between.
Have you heard the 800's plugged into a bottlehead crack? It may not provide you with quite the same all enveloping sound as a Woo 5, but it does the 800's real justice on a budget and sounds better, imo, than many of the "high end" amps I heard with them at CanJam. Synergy is key, but you don't necessarily have to break the bank to find it. Ears certainly vary, but based on the crack thread, I know I'm not alone in this opinion.