The Rotel RCD-991 was the successor to the previous flagship RCD-990 player. The 991 differed mostly in the transport used; the previous 990 uses a Philips CDM-9 Pro, whereas the 991 uses a Sony KSS-213. The Philips used in the 990 is perhaps a better sounding pickup, but suffered from reliability and availability problems (try finding one now).
The player operates smoothly and quietly. All function are accessible via the front panel; most can also be accessed via the remote control. It also offers the ability to adjust dither; a feature that I am not too familiar with. Internally, the player is comprised of a single PCB which is well laid out and of very high quality. Blackgate and Nichicon Muse caps are used prevalently; the Blackgates around the DACs, and the Muse caps in the output stage.
The biggest gains in modification to this player involve the output stage (I/V). Stock opamps used include the NE5534 for the I/V section, NE5532 for the anti-aliasing filter, and OPA2604s for the buffer/single-ended to balanced conversion. A choice of either single-ended (RCA) or balanced (XLR) outputs is offered. Modifications to my unit include THS4031s for the I/V stage, and AD8599 for the buffer stage. These resulted in much better sound, though the unit is quite good in stock form as well.
Rotel offered a tweaked version of this player in the RCD-991AE. All modifications to this that I am aware of were in the output stage.