Audiolover,
You're rceiver looks plenty capable of running the towers. The only thing you would gain from a new receiver is newer features. They change almost every year so playing "keep up" with receiver features can put you in the poor house. If the Onkyo does what you need it to do, I don't see the need to upgrade it at this point. I would look at sub/s next. Get the heavy bass load off of your receiver and the towers. After that, the center channel. You're di-pole rears are quite sufficient for surround, upgrade those at your leisure.
If after all that you feel the need for more wattage, then buy an amp. Emo makes a great amp at a great price. I don't hear any coloration from my Emo. The Bryston is a good amp as well, but at 3-4 x the price? Maybe it's worth it, don't know, haven't heard one. I hear McIntosh is great too, I still couldn't afford one.
Thats my two centavos.