Is the 5800 far inferior to the 5805?
I'm curious to pick one up used for kicks if available but they are probably to far behind to be terribly useful at this point in time
I think the opposite is true. The 5800 is also too old anyway imo, but you can use it as a power amp, it only weights 61 lbs so still manageable.
The 5805, as I said earlier, has the best DAC chip ever used in any AVR I know of (also the RZ-9). That by itself doesn't mean a lot but the fact that Denon would use such a high end DAC in an AVR, sent a message.
The boutique brand manufacturers would likely say right away (if questioned/challenged) that it is not the DAC that matters but the implementation. In my mind, that's an obvious excuse they use when the real reason must have been the cost. If they are so good in their implementation, why then not use the best available chip also, for a few dollars more? If go by "audible difference", we all know it doesn't matter if THD+N is 0.001% or 0.005% anyway don't we?
Again, I would rank the 5805 top overall because it likely has the best digital signal path among not only AVRs but also AVPs; and its power amp section is also excellent, even compared with separate power amps.
At the end of the day though, unless you can pick one up in top condition and cost well below $1,000 (say up to $600), it is not worth the hassle of the 60 to 100 lb weight just to use those things as power amps.