Subwoofer Newbie Looking for Recommendations

I am considering trying a sub with my Magnepan 20.7's. I have two options that I am considering, REL Gilbraltor G-2 and the Paradigm Sub 1. Any thoughts on which subwoofer will work best with the 20.7's?

Also, do you run a stereo set of interconnects from your pre to the subwoofer inputs? If you use two subwoofers do you run a left interconnect to one and the right to the other?
I have no direct experience, but I see you have no replies. Many panel speaker owners like Vandersteen 2Wq subs with their speakers. I do own these subs, and feel they are fast, extended and powerful.

Subwoofers offer some or all of the following connection options:

RCA sub-out from preamp to line-level RCA input on sub

Speaker-wire out from amp to speaker level inputs on sub

The Vandersteen uses a dedicated pair of speaker cables that attach to your amp speaker outputs along with the speaker cables to your mains. An in-line filter or crossover is inserted between the preamp output and the amp input (one for each channel).
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.
Either (and many others) will work equally well. The issue with subs is much less the sub and much more the setup. Depending on your electronics you'll have some connection options.

Many people believe that since bass frequencies are non-directional, that there is no benefit of using stereo connections with a pair of subs. If you fall into that category, you'd either use a Y-splitter on each cable (assuming there is no impedance matching issues) or if the sub provides outputs, you'd connect the subs in series.
(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
Love my Gallos with my Maggies. Don't know why they're so overlooked.
Secretguy, what Gallos are you using?
I recently added a pair of Vandersteen 2Wq subwoofers, along with the MH-5P battery-biased crossovers, into my system that includes Magnepan 3.7 panels.

The difference was nothing short of astounding.

The Vandersteens do blend seamlessly; offloading the amp really opened the amp's capabilities; and the adjustable Q helps you dial in just the right amount of bass, ranging from very tight and controlled, to loose and thumpy. (Tight and controlled, for me.)