I own the 3-channel RB993, which sounds much the same as the 991. Both these amps sound very similar to the current 5 channel RMB-1095. They are very powerful, with excellent deep bass and slightly rolled off extreme HF. I didn't test too many power amps when I got it 6 months ago, but compared to the limited internal amps of the Denon receiver I have, soundstaging and imaging (much wider now) are also much improved. Most NIB 991/993 amps will have been sold out, but I have seen some RB991 dealer demos going for AUD$1000 which is around US$500. I'm sure you can find one for less than this in the USA.
I also own an RB-993, which I paid $700 for used.
I agree that the RB-993 sounds very similar to the RB-991. The RB-993 has a third channel, a somewhat lower damping factor and a slightly larger power supply, but shares many components with the RB-991. I have been very pleased with the quality of the sound and the ability of my amplifier to handle high volumes without strain.
There are favorable reviews of the RB-991 in several audiophile publications. Rotel.com has links to some of these reviews or they can be found on ecoustics.com. This is the URL for the review links at the Rotel site http://www.rotel.com/html/reviews_amps.html
RB-991 amps generally sell used for between $500 and $600 on ebay or audiogon. This is about half of the MSRP of the current model that replaced it, the RB-1080. The RB-1080 has a 12 volt trigger that the RB-991 lacks.
I went to all mono-blocks and have an RB-991 and an RB-993 that I need to sell. Both in Like New condition with boxes and everything. I would think that they would bring over $500 and $650 on e-Bay, but I would take $1100 plus shipping if someone wanted the pair.
I own the RB991 coupled to the RSP1066. I love it. I found the only real difference between it and the new RB-1080 is the 12V trigger. I picked a new one here in Australia for about half the RRP of the RB-1080. I thoroughly recommend it. I also used the reviews at the Rotel site as a guide.
Like anything it depends
There were 2 iterations of the 991/993 if I remember correctly- 1 being the one without a detachable AC cord, and the other with the detachable AC cord and slightly higher rated transformer that was released later. So if you can find one of the later ones, in good condition, then $400 would be a steal. To get comparable performance would cost you 2.5 times that in the 1080!