I am considering a Velodyne DD-18...

Thanks, Dgarretson. In my current setup I run the B output from my Lamm LL2 out to a Sunfire Superjunior sub. The A output goes through an Art Audio Jota and from there out to Joseph 25Rm SigII speakers. The speakers claim +-2Db down to 32Hz but I measure a roll off much greater than that from about 40Hz and below. The Superjunior flattens that out down to 30Hz but can't keep it up in the sub-30 range and that's where I hope I can buld up some response.

Is placement in a corner/next to a wall critical?

Put the sub next to one of the speakers and use the amp outputs to the sub inputs.

Very nice system. You might compare both A & B options. Personally I prefer line-level inputs from pre to sub, even with tubes in the main amp. I think interconnects make a big difference in the quality of bass. I use a long DIY copper ribbon that's very good with LF extension and dynamics, and cheap to build. Room placement is less critical with the DD than with other subs. The spectrum analyzer, 8-band digital EQ, and granular phase settings, can compensate for most room anomalies. There is enough headroom in the sub amp to stack several bands of the equalizer on top of each other if necessary. Corner placement usually provides the most reinforcement, but not necessarily the cleanest bass. If you buy used, update to 2.0 control software via the Vel web site.

no one gonna mention the JL Labs, I mean, JL Audio f113?

it seems to have set a new standard in home subwoofers and is a bit cheaper i think.

Dgarretson , any more info on your IC? I'm using an inexpensive Monster M351 IC. Would I get any advantage to upgrade this?

BTW, the dd 15 is pretty darned great! Got mine Tuesday and it's still breaking in but have been more than happy with the investment. The Eagles on DTS was amazing with Timothy Schmidt's bass notes just providing a rock solid, tuneful foundation. Still working on breaking in and haven't done any torture tests on home theater yet but musically outstanding.

Don't be afraid to experiment with placement I actually got great results with the sub on the left wall firing out in front of the listening position, placement almost 90 degrees from the listening position.

The only thing I wish for is the ability for the SA to adjust phase or help you to determine as I've found that to be an enourmous benefit after getting a flat FR and integrating with your mains.