All three of my systems are run by great recievers (Denon 4802R, Denon 3808ci and an Onkyo 805) that are used as preamps and I have outboard stereo amps on all three systems, Did I need to do this. Hope not at all, but it was the fact that I can get stereo amps at insanely cheap prices that I figure why not. Have I noticed the difference. Yes but only slightly. I noticed slightly better clarity out of my near field rear channel speakers in my main system when I added a seperate stereo amp as compared to the Onkyo 805/ The difference wasnt significant to warrant the purchase, but as I tell my GF it's a cheaper hobby than a mistress.
I would say if you have the stereo amp hook it up, won't hurt anything and may give you more headroom and relieve your recievers internal amp with more power to drive the other speakers hooked up. Looking back I would now consider the outboard amps if I had a huge HT room (I don't) and had spent that money on room acoustics (which would give you far better improvemnets that the small incrimental improvements with an outboard amp.
