Mostly it's in the numbering series but NR is a slimline avr lineup, SR full sized avrs, PM is an integrated amp line up (2ch). Keep in mind modern Marantz has been married to Denon for quite a while and share a lot, and often Denon is better priced for the same feature set. Ok so what about the different line ups for denon? Seems just the 2. The s and x. X being the premium?