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

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

larstutor wrote:

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?

Are you using the Mirage cones for seating the speakers on the floor? Otherwise if they're just sitting flat on it that could reduce the bass. My mid-'90s M5si's have bass wherever you put them. The OMD-15s are capable of low 30s bass, but can drive you crazy trying to find it.

When it comes to sub power and depth, diameter isn't the only factor in making a strong wave; excursion matters, too. Total volume displacement is the diameter times the maximum excursion. Bob Carver and Sunfire practically invented the high excursion subwoofer driver, with that big reverse roll surround that would allow a 1" excursion when 1/4" was still considered extreme. Of course going up in diameter *plus* the high excursion makes it deeper and stronger yet, but Sunfire claims to extend into the 20s and the reviews I've read of their subs indicate that they tend to go as deep as they claim.

I'm sorry if I gave the wrong impression; I haven't heard the HRS8 so I can't comment on room-filling capability. But I have some reason to trust them as I've heard other very compact Sunfire subs. 

As for the NHT Classic Ten and Energy ESW-8, I think you're shooting too low. The Classic Ten was designed to mate with a dual-woofer tower and the ESW-8 with the Energy Take 5 miniature system. If you want to add a real bottom end to the OMD-15s (which I think is worth doing), you'll have to up the ante unless you luck into a deal. 

Consider the SVS SB-1000, a 12" sealed enclosure (best for music) powered sub that's $499 in black ash or $599 in piano black gloss. This one should be plenty fast and elevate your OMD-15s to a higher (and lower :-) ) level. Lots of positive professional reviews.

• Goodsound (Soundstage)
  
• AudioFi.net
  
• Sound and Vision
  
• About.com
  

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.