There is no single best answer for using a separate amp with a receiver.
There is one group that argues for the improved power output of a separate amp. Amps are generally more powerful than a receiver, but a near flagship model of receiver from Denon, Yamaha, HK, NAD, Rotel, Pioneer etc will generally be rated at 75 to 100 wpc, with most separate amps having 100 to 200 wpc... a slight 3 or 4 db increase in sound volume, so the separate amp is not an absolute improvement.
As a note, not all receivers can produce their rated power "all channels driven", yet how much power is really needed for the surround channels? Thus the idea of using a less powerful receiver (40 or 50 wpc) for the surrounds with a large amp for the main speakers.
There is another group that argues the noise floor (background noise of the unit) is lowered with separate amps. That argument is negated by the fact that modern receivers are actually well designed and very quite {hence the desire to use the receiver as a preamp}, and the additional cables, and RCA connections could add to the noise floor.
Because the quality of receivers has improved, purchasing a very high quality amp may be a good idea, and then, as technology evolves, purchasing an inexpensive receiver every few years to keep up with the changes may be a very cost effective idea.
The argument against separates has been stated by the Audioholics editors, who have been promoting the ideat that a well designed flag ship or near flag ship receiver is a viable platform for HT, with sufficient power and features to negate the need for separates.
Then again, remember the old adage, the boy with the most toys wins.
