Check out the REL R-205...
Yes, I believe you're on the right track. Third the REL R-series, they've worked great for me.
That said, the SPL 800R would be a nice pick as well; I've got their bigger brother, the SPL 1000R (x2), in my living room HT/2 channel system and am very pleased - fine for HT and Music. The room correction feature, remote, and music genre setting capabilities really give one a lot of versatility.
Re: using pre-outs or speaker tap, try both and see which one works best to your ears. For me, utilising the speaker level taps sounds slightly better.
Definitely go with a sealed subbetter damping and hopefully a lower Q. Don't overlook kits. Sealed sub kits are available for well under $1k, very easy to assemble, and will give you higher performance for your dollar.
Powered subs have line level inputs. But to get the integration between the sub and mains correct you need to have the crossover set correctly. If your preamp has a subwoofer crossover and sub-out you can do it from there, but unless you're using an AV pre this probably isn't the case. So, you'll need to use the crossover in the sub (or a separate external x/o). Pre-out goes to the sub, crossover sends the mid/hi to the amp from the sub's high out. Crossover frequency should be set at about one octave above the -3 dB point of the mains. You need to have that much overlap for a linear response.
Be very careful in your selection of a sub. Too many are pseudo-subs that are barely flat to 30 Hz. To be useful, a sub needs to do 25 Hz (-3dB or better). This point can be debated, but the low A on a piano goes down to 27.5 Hz, the low C is 32.7 Hz, so flat to 30 seems reasonable.
Adding a sub will give other benefits besides low end extension. It takes a lot of stress off your mains (and amps) helping to reduce distortion and improve the lower midrange.
What about Vandersteen 2Wq a very "musical" sub that lets you set the hi-pass filter value to blend with your speakers and also has db matching to your speakers and adjustable" Q" from tight (jazz) to bomb explosion, it's best at the jazz end though. The signal is coming from your amp so it takes on the Jolida's characteristics.
you have the right idea re sub addition. fwiw, i use the same spkr outs to run to my MK powered sub which has all the controls built into it. i run a thinner gauge wire from the binding posts from a different position on the 5 way posts to the sub. exlnt results. i have the jolida 502b and yes these 'smaller' tube amps do benefit from some s.s. muscle in the deep bass. the current sent to the sub is miniscule and heavy spkr cable hardly seems justified in this sense. jmo
I line up behind Samhar on the Vandy 2Wq. Have been using mine with Magneplanar 1.6QRs in a two channel only system for the last nine years and am completely happy with every aspect of it's performance. Make sure you understand what you're getting into with the 2Wq as it uses a somewhat unconventional but extremely effective hookup method.
My favorite sub (I've only owned two) is The Anthony Gallo MPS. It's the two-piece cylindrical one, not the sphere and not the cheaper "cannon."
All their promotional material mentioned the "texture" of deep bass, and I have to agree that it has an exceptionally textured and rich presentation. 240W amp, 11" diameter, HUGE bass, but refined, highly detailed, fast, and not boomy in the least. I even used it in rooms that were supposedly too big and never really ran out of steam.
They may be out of production, but you can get a used one for less than $500.00.
I owned the Velodyne SPL-800II and enjoyed it. The sub was easy to integrate with my system which is a plus. But for deeper, cleaner bass I purchased a James EMB-1000. Another sub which is easy to integrate. This thing can rattle the windows with no strain. It's a sealed unit also.
I haven't read all of the answers but IMO use the variable preamp outs instead of speaker outputs.
I second what Pacific_island and Samhar have said.
The Vandersteen 2wq Subs are sealed boxes (not ported) and so the bass is real and tight (not bloated and full like the ported designs).
Having 2 of them as a stereo pair not only will give you the bass you want, but will help fix any room resonant problems as it will 'pressurise' the acoustic space if positioned in 2 opposing corners. They also will help eliminate intermodulation distortion in your main speakers' woofers.
I had the REL Stadium II for 10 years and the Vandersteens are in a different league.
Sub woofers are not as easy to integrate into a system as some of the posts would have you believe.Of course these subs have become easier to work with over the years but they are still not ideal.The more expensive subs offer more integrating options.The lower priced subs can and usually will degrade
the lower mids.You may receive enhanced bass but at a musical cost.