My guess is either of these Rotel amps, if in good condition, may be an upgrade relative to the Peachtree amp, but you may enjoy an even greater difference by saving for an amp a little more upscale and newer.
I have owned several Rotel amps in the past, including Rotel 980BX, 981 and 993. All of these amps were purchased used and resold after a few years for >80% of my investment. In my opinion, Rotel makes nice sounding, reliable amps, but there may be better choices if you are patient. I eventually replaced the Rotel 981 amp with a Parasound Halo A23, which to my ears was a noticeable improvement in driving my SongTowers.
Also consider that the Rotel 980 and 990 amps are now 15 to 20 years old.
http://www.rotel.com/NA/Products/ProductArchives.htm These amps are reaching an age where capacitors and certain other components may fail or degrade sound quality. This is a greater concern if the prior owner operated these amps for prolonged periods without adequate cooling/ventilation or at high ambient temperatures.
My expectation is that unless you play your system at very loud levels, you will not hear much difference between a Rotel 980 and Rotel 990 (assuming both are in good condition) driving your SongTowers. Also, you are more likely to be able to recover more of your investment buying a good condition 980 for half the price of a good condition 990, if you trade-up in the future.
If you could take an amp on approval or buy it with a right to return or exchange the amp in a few days, this may be an even better option.
YMMV.