Subwoofers.... one 10" or two 8"?

The take away is that SPL is key. And fortunately most music doesn't contain much bass.

Exactly, especially given that 2-channel music is the OP's primary use of the system. I have used a Mirage LF-150 (10" x 150w) and an LF-100 (8" x 100w). Believe it or not, using the Stereophile test & demo CD, both subs produced clean, clear sub-30 Hz warble tones easily audible from the listening position about 8-10 feet from the subs. They might not be the best thing for crashes and explosions for HT in a large room, but for 2-ch. stereo they did surprisingly well. The more upscale sealed w/ passiave radiators Mirage MM8s do better (on transient response, clarity, etc.) but don't go as deep as even the modest LF-100.

I've also heard the SVS SB12 with full-range speakers in a fairly large media room and it acquitted itself well on 2-channel music. I think it's a good candidate for the OP. 

How good are Dynaudio subs? Anyone tried?

I have had over the yrs probly 10 different subs . The original 8" sunfires with my mg3a sounded killer .. I had a pair  of the better 10" velos granite looking base with kefs they were imo not as good as the sunfires anong other velos, a single rel few yrs back 12" it was nice i have a hrs 12" sunfire on spikes now it gets boomy if its not placed right . I would like 2 10" in my room but honestly there are many times i dont even use the sub. There are many things at play with a sub regarding placement , straddling corners etc. but i never knew how good the low end sounded until i started working on room acoustics . Regarding small subs and high distortion i can't comment , i only know what sounds good in my setups over the yrs . The only 15" i ever had is the velodyne i have a model i can’t remember at the moment it was around a g retail is in my ht room its huge and rattles the house but tight is it not . 2-8" = 😬 2-10" = 😃 2-12" = is overkill in a small med room . Jmho .. Treat that room with some bass traps of some kind you’ll be surprised.👍😄

It is good to read what I have known for many, many years now regarding REL and Sunfire subwoofer(s).

This set up is not in a dedicated listening room but rather in our living room and  I don't think my wife is going to stand for bass traps. Anyone else experience a similar problem? (lol)