Since you have not indicated a budget for your amplifier upgrade plan, or stated a ratio of music to HT usage, I'll simply throw out my ideas, recognizing in advance that it may not fit your requirements. Let me open this thread by making two comments:
1. IMO, there are almost no 7-channel amps on the market that I can personally recommend, since the good ones are very expensive (typically $5k or more), and most of the cheap ones are not worth owning if you have good main speakers (which you do).
2. The main speakers in your system (the left and right front channels, and maybe the center channel) require more power than the surround speakers. Therefore, you may want to consider buying a good 2-channel (or maybe 3-channel) amp for the front speakers, and a 4-channel or 5-channel amp with less power for the surround speakers.
In building my own system, which I use about 85-90% of the time for music and 10-15% for HT, I opted to use two separate amps: a Bryston 4B-ST for the two main front speakers, and a Bryston 5B-ST for the center channel and rear surrounds. I have been extremely satisfied with this setup, and you may want to consider a similar arrangement. If Bryston amps will fit in your budget (either used or new), you could pair a 4B-ST (2x250 wpc) amp with a 9B-ST (5x150 wpc) amp, or a 6B-ST (3x250 wpc) amp with a 8B-ST (4x150 wpc) amp. (Bryston amps are conservatively rated, and typically provide 15-20% more power than their published specs. The newer "SST" versions of the Bryston amps I've mentioned have somewhat higher published power spec's, but are seldom found on the used market.) Either of these amp setups will give you excellent, flexible amplification, as well as Bryston's excellent, transferrable warranty (20 years, or remaining balance thereof).
If your budget can't manage two Bryston amps, you could still follow this same 2-amplifier setup for less money by choosing amps from Rotel, Parasound, and Adcom, all of which offer very good value (cost-to-performance ratio). (I particularly like the Rotel line of products -- they offer really excellent performance for the price.)
Having said all this, I'd now like to ask you a question: why do you particularly want to go to a 7.1 channel system? My intent here is NOT to criticize your personal judgement or preferences, but rather to generate some further discussion. I have heard enough home theater systems, and helped install a number, that I am not at all convinced that 7.1 channel HT surround offers any advantages over a very good 5.1 channel setup. Infact, since the 7.1 channel system requires two more channels of amplification, and additional speakers, one may be required to compromise on the quality, and actually wind up with a LESS good system than buying a good 5.1 channel.
Bryston had several very interesting articles on their newsletter site, making the point that Hollywood is a LONG way from releasing fully discrete 7.1 channel DVD's -- any may not ever do so in any quantity. Further, most home theater systems are in listening rooms that are only large enough to properly accommodate 5.1 channels of audio, much less 7.1 channels. But rather than reiterate all of the points in the Bryston commentary, I suggest you read the articles yourself and use them as a basis for making an informed decision about how to upgrade your system. Here are the two links to the Bryston articles:
(see the article titled "Encode vs. Decode Dilemma")
(see the lead article)