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

Thanks all for the good information and suggestions.

I owned a pair of OMD-15s for five years and had a pair of Mirage MM8 8” compact subs with them for at least three of those years. The OMD-15s are pretty picky about getting decent bass. It requires a solid (e.g., masonry) floor and reasonable proximity to room boundaries to get meaningful bass below around 42 Hz. My pair was in the living room on a suspended wooden floor and I couldn’t get the wide cones to penetrate the carpet all the way to the floor. I thought about getting Soundocity outriggers to aid in seating the speakers, but I came across some closeout specials for the MM8s and bought them instead.

If I’d had more money I might have gotten a couple of bigger subs, but that’s what I got and the advantage of the MM8s is that they were *very* quick and blended easily with the OMDs. The downside is that they were only good down to around 36 Hz. If I could have gotten a sub or two that extended into the 20s, this rig would have really excelled on a wide range of music including one of my favorites—bombastic large-scale orchestral pieces such as The Planets, Scheherazade, etc.

Which sub size you go for depends on how much grunt and bass extension you can get out of the OMD-15s. Can you spike them clear through to the floor and ensure that there’s no wobble? Can you move them around and find a balance between imaging and bass reinforcement from room boundaries? Then they can reach into the 30s and you would probably want a 10” or 12” sub and use a lower crossover point. If your bass is disappearing around 45-50 Hz you may need to stick with a pair of 8-inchers to get a good blend at a higher crossover point.

A third option if you can spring for it, would be to get a pair of Sunfire HRS8s, an 8” subwoofer with a very high-excursion driver that can realistically reach down into the 20s.
http://www.sunfire.com/productdetail.asp?id=17

The JL Audio Dominion d108 is another 8" sub that reaches down to around 30 Hz.
http://www.jlaudio.com/d108-ash-home-audio-dominion-powered-subwoofers-96281

My OMD-15s are sitting a a very solid hardwood floor over a plywood sub-flloor  built on a concrete slab.  Contact between the speakers and floor is rock steady.  I wonder if I'd get more bass by putting the speakers on feet to isolate from the floor.  It's hard to imagine an 8" subwoofer going down into the 20s.  How does the HRS8 sound at moderate listening levels?

My 8" REL t108 II is absolutely a true sub, and I use it with a REL 10" which is from the same era. Speaker size is less important than the overall design of course, and although the 8" REL is rated to 23hz or so, it works as a fine musical sub. I recently bought a mint Mackie HRS120 sub designed for studio use (to use in my studio…) with a 12" driver and a 12" passive radiator, it weighs 95 lbs, has a 400 watt A/B amp and its -1.5db point is around 19hz with very low distortion. Used, under 300 bucks. Yeah.

x2 on the above comments.rels are good subs.Although i am often perplexed why I don't see more people running the better sunfires they a great subs.The 8" sunfire back in the 90s was a beast for its size i had 2 with mg3a and cj it was great (treated room).I have a hsu 12 and it will peel your face off if you want it too.It smokes the velo 15" i have in the ht room ..Again and again i say it invest in treatment it will change the game in your room...gaarrunnttteeeed . lol any good sub will sound way better with treament I personnally have seen it again and again. again my humble opinion..