My room is similar in size to yours, although not quite as robust (wood framing and a crawlspace below), and I've got the original LFM-1 in my space. It does a very nice job, and integrates well with my Paradigm Studio/60's (which likely get down about as low as your Spendor S9's). No muddy sound and no "hey, look, I'm a sub over here in the corner!" sense that you can get from some subs that don't integrate well, but enough muscle to give strong bass performance.

If I were shopping for a sub for my room right now, I'd probably be looking at either the LFM-1 Compact (similar low frequency extension as my LFM-1 with about 3dB less maximum output due to the smaller driver) or the LFM-1 Plus (same basica design as my LFM-1, but with the ability to plug one port and get extension down to around 18Hz. Both are on sale at the moment, it looks like.