It would be interesting to get some listening comparisons done with the Denons using inexpensive and commodity amps. The Denons can do well from some unspectacular or commodity amps. But they're still particular. They need a certain minimum amount of power and reserve (I'm guessing more related to current given their low impedance). Once they have that minimum they are in the game.
It would be interesting to find examples of cheap portable amps that work with the Denon. How cheap can you go and still keep the Denon happy?
What are some home receivers that work well and ones that don't?
I've tried my Denon D2000 in a few home size CD players and been surprised by a few of them. You don't expect to have much of a headphone amp in a home CD player. But a few of them can deliver for the Denon.
My baseline for minimally acceptable amplification for the Denon is something that is a good step up from my iPod Classic. The iPod can make the D2000 play music and you can listen to it and it will sound OK, but it isn't doing it well enough. Better amplification gets you more dynamic headroom (more punch), more soundstage, better control of the bass, and things like that. It's a difference that is easily noticeable if you're used to listening to audio gear and are familiar with what you are listening to.
My general experience is that battery powered devices that try to conserve power (get maximum play time from the battery) don't do so well with the Denon. Devices powered by USB or FireWire won't have enough oomph either. Devices powered by a wall plug will probably have a better chance of sounding good with the Denon.