I've never used 'SR', but own a lot of records that have it. It seems to put a bit of a blanket over the sound. 'A' is a totally different ball of wax. It is thin (clenched bass especially), metally, and dynamically flat sounding. Recordings that use this NR either fit aesthetically with the above described qualities (80's pop music), or were made in the 70's using huge sounding equipment that sort of balanced things out. DBX on a 4-track sounds like shit, but on a 1" 16-track is a more transparent option than Dolby A. The thing about DBX is that it's a full band compression-expansion system, so the tonality of the sound doesn't seem to get as skewed as with Dolby. Regardless of what NR you use, don't be putting the tape in the red cause it brings out the worst in the NR. Lower your deck's reference level by at least 3db according to tape stock.
The guy who wrote the fancy pantsy microphone book that came out a few years ago told me DBX NR was standard for classical 2-track masters before digital took over.
Nicholas