I've used the Behringer A500 both in stereo and bridged modes and there is no lack of detail going from the internal amps of my Yamaha RX-V2500 nor is there any difference compared to an Adcom GFA-5500 I own as well.
My experience is similar to Nuglet's post. The A500s (particularly in bridged mode) allowed for plenty of headroom and became almost impossible to drive to audible distortion. From many other posts I've come across, the Crown and QSC amps are also great performers for the money. Amplifiers should rarely (if ever) alter the sound, the only time being when they are driven to audible levels of distortion.
Now as for the fan, well that's a different story...
-TD
ps
Also of note, the A500s do not have a fan. The next step up in the Behringer line does..