Well, I have this Sansui amp/preamp setup as well which is usually used with the CD player and speakers, which I sometimes hook up the 555 to. And that is a really bassy amp, so if I want a warm, bassy sound then I usually hook it up to that amp.
The Denon home theatre amp (can't remember exactly which model) is probably what they would call more balanced or accurate. It doesn't have as much bass but crisper, brighter treble. I use that just when I'm using the 55 for DVD's or if I want to listen to music with more details which I feel the other amp might be drowning out with the excess bass.
I also use the HD555 straight from headphone out of my iPod and it sounds great like that too, albeit lacking the bass that the amp provides, but brighter treble in ways. Sometimes, I find that a little extra bass just makes the music fuller and complete sounding so I usually amp it unless I am listening in a different room to where the amp is or something...
Oh, and on the whole, the 555 is nearly just as much fun without an amp as it is without...amping just gives it some juice. I've read that HD555 is the one to go for if you're planning to use without an amp as well. The HD595 apparently need amping to shine.