Uh-oh, don't you hate when you get five completely different answers to a question?
I echoe Mr. T's question of what specific equipment you have, because that will determine which response is right.
Most HT receivers share one power supply for all output channels, which means when all channels are being driven you could be reaching the current limit of your power supply and hitting the limit - which would compress your peaks and take away the punch.
By using an external amp for the mains, you're dividing the output current of your HT receiver's power supply over fewer channels, which means you're less likely to hit the current limits. This means more dynamic power and overhead - more power to share all around.
But that's only true under the assumptions that a) you're driving full range speakers as surrounds (driving channels full range takes more power), b) your HT receiver is built to give a higher power rating per channel than with all channels driven, c) the amp you choose for your external 2 channel is as good or better than your HT receiver.
As for level matching, you can use a level meter as hopjohn recommends, or you can "wing it" by ear. You set the volume/gain control of the external amp one time, and leave then leave it.
Gotta go with zumbo about the powered sub. If it's a decent powered sub, then its amp is adequate. If it's not a decent powered sub, then using a different amp won't get you anything because the amps are often the best component on powered subs.