If you are considering an amplifier, one thing to look for is amplifier gain. Amplifiers have a signal gain in the 26-32db range. The Emotiva XPA-1 gain is 29 db, which is decently in the middle. I know some of the Wyred 4 Sound amps have a gain as much as 30.7db. If you get an amp with a high gain, the Rotel preamp does not have to work as hard to push a higher voltage signal - this can help keep the signal distortion lower.
If you consider a sub, the important thing to make sure is that you strip out all the low frequencies from your main GoldenEar speakers. If you’re just adding a sub to help with bass and not removing the bass signals from the main speakers, it will do nothing to help your high volume distortion because the amp is still pushing a full-range signal.
You can either apply a passive crossover on the GoldenEar speaker wire (which is not really the best solution), or you can use some sort of active crossover to ensure the Rotel amp is not driving a full-range signal. You can use the "pre-out/main-in" processor loop connections on the lower part of your Rotel. The "main-in" inputs go straight to the internal amp. If you can’t find a good active crossover or you don’t want to run long lengths of RCA cable back and forth from your subwoofer, you can try using a pro-audio crossover, like a Rane AC22 or AC22B (lot’s of pro audio crossovers on ebay for less than $400 -- Rane, dbx, etc.). You’ll have to get some RCA-to-XLR or RCA-to-TRS cables to use this option.