I think Jeffrey has a good point. As none of these are out yet, there's no way to compare audio performance. As far as features, the 950 gives up a few features (THX being a big one, and one that isn't really much of a loss in the greater scheme of things) but offers the benefit of separates that you pointed out. The upgrade to the Denon 5800 will be expensive and will not add full 5803 functionality, which doesn't make me real warm and fuzzy about future Denon upgrades (it's nice that they offered at least some upgrade to the 5800, but the price is steep -- almost what a 950 will cost). The Pioneer 49TX looks like a fairly impressive piece of gear, but I think the THX Ultra 2 is a major reason for the $4200 list price -- paying a lot extra for little more than a name and a fixed 80Hz crossover.

What about mixing things up a bit? You mentioned a used MC-1 and used amplification. The used MC-1 will cost you close to what a new Model 950 will -- why not consider used amps and a new 950? Just another idea...

Best of luck with your search!

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Gonk