Subwoofer Newbie Looking for Recommendations

thx; i have gotten a number of dealer recommendations to try the big REL's, like their G-2

Sorry, I didn't see this thread until today. I have heard Mag 20.1s paired with a pair of JL Fathom F212s. The sound was choherent and seamless. The sub was undetectable as a separate entity while providing strength, extension, and dynamics from 40 Hz down to 20 or slightly below.

JLs are crazy fast and linear over a broad frequency range. I'm not sure what your budget is, but JL's new E-series makes their subs more affordable than ever.

Yes, JL are fabulous and have to be the best out there. That's where my money will go.

(assuming there is no impedance matching issues)
when you hook up subs at line level from your tube pre amp
you will drag down the performance by loading your pre amp into a lower input Imp.
Speaker level as Bondmap suggested works best in my experience. like Rel and Vandersteen
The other advantage of the Vandersteen is their easily adjustable High Pass unloads the amp from the lowest frequencies which lowers the whole systems distortion dramatically improves transparency and clarity.
JohnnyR Vandersteen dealer