Why so many JL Audio Fathom subs for sale I wonder

Usually highly reviewed = more sales.
More sales = more re-sales.

Pretty simple math.

IME, it's difficult (tho not impossible) to properly integrate subwoofers into a 2 channel music system by ear. Many, many moons ago I tried and failed. I didn't revisit the idea for three decades. Now, I use Audyssey and it's simple and straightforward. However, I doubt that many people here want to go that route. Instead, they try manual set-up and, not surprisingly IME, are disappointed with the results and sell the subs.

As to JL specifically, I suspect it's because JL and REL share the biggest piece of the high end audio market for subwoofers. At any given time, I'd think that one or the other would dominate the re-sale listings here. OTOH, given the lower price point offerings, I'd think that you might see more Velodyne products on e-bay, or the like.

I own Rythmik subs, but I've auditioned JL extensively and think that (provided they're properly set-up) they are outstanding performers.

Marty

I use my f113 primarily for HT and it is quite a seamless affair.

Using a sub for music is as others have noted not a simple job. I always hated finding the right point because there never seemed to be one... what seemed ok for one track was too much or too little for another. I still have trouble dialing in the active bass section of my in-law's Martin Logan Ethos.

But with my Thiel SS2/2.2s and an S1 integrator, it's been a breeze. Jim Thiel is a genius. He took the guesswork out of integrating a subwoofer. It's all a matter of measuring how far the sub is from its nearest walls (and dialing those on the knobs in the subwoofer) and reading off the specs of the speakers and power amp (-3dB lowest frequency, gain, sealed/ported) and telling the S1 box what those numbers are.

I agree with the Thiel post. However, I understand the integrator, which could be used with any speaker, is no longer made.

Martin
What did you think of the JL's you heard?