I'll add one caveat to John's post. There are some amps that when their volume controls are turned all the way up, they go past unity, and the pre-amp stage is actually boosting the voltage that's being delivered to the amplifier.
You're receiver should have a manual test test tone mode. Activate that for your main channels, and then with the volume control on this external amp set at half, move to the one of these additional channels. Switch back and forth, comparing the two (if you have a SPL meter, it'll be even easier). You're don't have to be exact if you'll be using the auto setup later, but the idea is to bring up the volume control on the amp until the speakers it is driving seem about even in output compared to the mains. If the volume control is almost all the way up, turn it the rest of the way, but if you're only at something like 3/4 and already as loud as the mains stop there.
So what John said is right in most cases, but not all. Just so you're aware.
