Amplifier to complete a 7.1 channel system w/great stereo music performance?

Because you're still using the preamp section of the Rotel, which isn't on the level of a good dedicated stereo preamp and so will continue to be a bottleneck to achieving better sound.  One of the big surprises many new audiophiles have as they try different equipment is how big an impact the preamplifier has on the sound of a system.  Most of us come into this hobby underestimating its importance -- me included.  You've got some very good speakers that will appreciate and allow you to hear the difference as you add higher quality upstream components. 

When I added a better stereo preamp and got the Rotel out of the chain for 2-channel, everything sounded richer, fuller, and the soundstage expanded in all directions.  Individual instruments and vocals also took on an added heft, solidity, and presence that sounded much more real and lifelike rather than relatively thin-sounding facimilies.  On the top end there was more detail flushed out, yet the treble sounded less brittle and more refined overall.  I just mention this so you have an idea of the level of improvement you might expect to hear by adding a better preamp, and a better stereo amp will likely also add substantial benefits as well.  In this way a good integrated amp can kill two very big and important birds with one stone, while separates give you more flexibility and potentially better sound albeit likely at higher cost.  Either way, your 2-channel listening experience should become much more enjoyable and rewarding.  Do research and read reviews, listen to as much stuff as you can, and enjoy the journey. 

Just wrote a long response and got kicked out and lost it.  ARGH!!!  So I'm making this one brief(er).  I've been down this same road with a Rotel RSP980, so here are some thoughts from my experience. 

Adding a stereo preamp is just as -- and maybe even more -- important than an amp if 2-channel is your priority.  Just in case you're not aware, you can seamlessly add both into your existing HT setup without having to swap cables.

With your budget you can 1) add an amp, 2) add a preamp, 3) add both, or 4) add an integrated.  If $2000 is all you want to spend I'd go with #4 or #3.

The most cost effective would be the integrated route.  A used Hegel H160 (available here now) or used Parasound Halo Integrated would be big upgrades over what you have, and both have a HT bypass to easily integrate with your system (ask if you're not familiar with this). 

You could do good separates if you up your budget just a bit using this equipment:  There's a nice McCormack DNA 0.5 available for $700 and a Hegel P20 for $1650, which would probably be a bit of an upgrade over the integrated options above but would also require another pair of interconnects.

Next, if you're not opposed to spending more later, would be to spend the $2k on a good stereo preamp and add an amp later.  Reason for preamp first is that your amp is decent and you'll probably benefit more from a good pre that gets those low and sensitive line-level stereo signals out of the electronic morass of your prepro.  In my experience, and many others here agree, changing preamps actually makes a bigger difference than changing amps -- of course YMMV.  Anyway, you could go with the excellent P20 above that by itself will likely yield a big improvement in your stereo performance, or go with a Rowland Capri or Parasound JC-2, all of which have HT bypass and are available here now.  Can't see going wrong with any of these great preamps. 

My my last option would be to do what you're asking about (sorry) and get a $2000 stereo amp first and then add a good stereo pre later.  The A21 is a very nice amp that you can get used for $1500, and there are also Bryston 7B SST monoblocks available here for just a little more.  If it's me, there's a McCormack DNA 1 with Steve McCormack's Gold revisions within your budget that I would absolutely jump on (I have a DNA 0.5 with A revisions that are a step down from Gold I believe).  Again, can't go wrong with any of these IMHO. 

Yes, this is a shorter version of my prior post believe it or not.  My last piece of advice for these types of electronics is to buy used.  You'll get much more performance per dollar, and if you decide to upgrade later you'll take much less of a financial hit.  Whew.  Hope this helps and best of luck.  Peace out.