Anybody prefer JL Audio or Velodyne subs for rock?

Get the DD18.
http://www.velodyne.com/newsletter/volumes/v14/article_1.html

http://www.velodyne.com/newsletter/volumes/v14/article_8.html

http://www.velodyne.com/newsletter/volumes/v14/article_4.html

Both subs you mention are tremendous performers - probably overkill for the vast majority of 2 channel applications. I really doubt that you'd hear much difference. You won't go wrong either way...provided that you get them well integrated with your mains and that your room can handle the output. I use a Velodyne SMS-1 sub controller to help integration. It's reasonably priced and it works like a charm.

BTW, I haven't heard the SVS that Knownothing recommends, but they have a great rep and test off the charts. They offer in-home trial, and if listening bears out the specs, these subs would appear to be hard to beat for the money.

As long as you have a well built sub (both the JL and Velo are), then the sound that you end up with is 90% setup related. Whether it sounds tight, fast, authoritative, etc, has to do with your room's acoustics, and how well the sub works within the variables that the room throws at it.

Integrating a sub takes some work, but is worth the effort of course. The Velodyne SMS-1 recommendation is a good one if you go with the JL.

Consider a REL B1 as well.

Frankly any one of the SVS PB13Ultra, JL F113 or the Velodyne DD18 are going to be excellent in either mono or stereo (it doesn't matter much but you may get a more even in room sound if you have two subs to play with and you have the liberty of placing them in the best spot). I'd guess that the DD18 will beat the smaller drivers in terms of absolute SPL.

The advantage of the PBUltra13 is you can play with the ports (all three blocked, or any or all three open) which will change the "Q" or resonance of the sub - this may help in integration with the bass of your speakers (I am not sure what sort of response the Klipsch La Scalas will have but they look like bass bins or folded horn designs so a resonant sub with higher Q will integrate best. If you can find group delay measurements on the Klispch or an anechoic frequency resposne then it will be easier to judge what would work optimally. Suffice to say that getting a very tight overdamped musical sub may cost you a lot more money than you need to spend and also give you lower SPL then something with greater resonance or group delay)

Running Cornwalls with a McSystem. I wanted to experiment with a sub. Considered the Trinity from Def Tech and JL Audio F113. Wound up with a Klipsch RT-10D for less than 1/3 of the cost of the others. May want to look at Epik subwoofers too.
If cost doesn't matter, go with two JL F113's.