Subwoofer for classical music listener

Here's my 2 cents and it may just be worth that!
I've done a lot of research on subs and am an advocate of REL's. My system is dual purpose, 2-channel music and 5.5 channel home theater. When I say 5.5, meaning I have 5 subwoofers. A loudspeaker is only designed to generate frequencies to a certain level, typically around 40 Hz for floor standing and 60 Hz for quality bookshelf's. Therefore, why not allow every channel you have produce a full range from 20-20K? Anyone who does an A/B of a system with and w/o a sub quickly discovers something missing without the sub. We live in a full range world. If accurate reproduction of a musical performance is what we're after, then it's important to reproduce everything that was present during the original performance.
As far as REL's are concerned, they are integrated using a high level signal tapped directly from the speaker tap of the amp. This way the sub gets the signal at the same exact time and the same exact signature at the corresponding speaker. Setting the crossover and volume can be a bit tricky however, once set correctly, it's a seamless integration. You shouldn't hear your subwoofer. You should hear the system!
A subwoofer should cost roughly what a loudspeaker cost, or more! It's much more complex and it's the subwoofers job to integrate in the system, not the pre-amps or external crossover's!
I encourage anyone looking to buy a sub, to consider a REL. Don't just look at the pretty box, READ the technology behind it, how it's integrated, and why. Most other subs rely on the pre-amp's crossover or a separate external crossover. If buying extra gear to display is your thing, go for it. If you want a system that is truly integrated, synergistic, and sounds like its all "one" system, read about how the REL works....

I've found out the hard way that I'm extremely limited in what kind of subwoofer I can match to my NuForce class D monoblock amps.  If you've followed this thread you've read that the Vandersteen sub I bought was incompatible with my amps and could have damaged them.  (Thank goodness they're okay.)
Evidently I need a sub which can accept a "floating output" sub. I can't hook up subs to the same posts as the speakers use, which was the case with the Vandersteen.
I've  been told some REL models are compatible but I don't know which ones.
Does anyone know of any subs that are compatible with my type of amps.
People suggest I should change amps instead, but I'm really happy with how perfectly the NuForce amps match with my system.

I'm running a REI Stentor with my DALI Epicon 2's.  
It took me a while to dial it in.  Solo Harpsichord for some reason would always expose if I did not have it accurately dialed in.  This holds true for my DALIs and my previously owned Dunlavy SCIVs.
Once properly integrated, besides what a subwoofer does for piano, Janacek and Stravinsky, it really brings in the 'hall' of the recorded venue and more 'wood' from wood based instruments if that makes any sense.

Rvpiano 6-3-2017
Does anyone know of any subs that are compatible with my type of amps.

I doubt that a sub can be found that would be suitable for connection to the outputs of your amplifiers.  But a considerable majority of subs provide line-level inputs, in addition to or instead of speaker-level inputs.  That would include most and possibly all of those that have been suggested by the others, aside from the 2wq.

So the approach I would suggest is to choose a sub which provides line-level inputs, and connect it to the outputs of your preamp.  If the preamp just provides a single pair of output jacks you could connect it to both the sub and the NuForce amps via a pair of Audioquest "hard y-adapters," or equivalent.

In doing so, however, from a sonic standpoint it is important to be sure that the preamp has a low enough output impedance to be able to drive the combined input impedances of the amp and the sub with good results, and also to be able to drive the capacitances of both sets of interconnects.  In most cases that same caveat would apply even if the preamp provides two sets of output jacks, and a y-adapter is not necessary, since in most such cases the two sets of output jacks are wired directly together inside the rear panel, and driven by the same output stage.

The line-level input impedances of many subs are relatively low, often between 5K and 20K, which can make that issue a particular concern.  And the output impedance of some preamps, especially tube-based preamps, often rises to much higher values at deep bass frequencies than the specified nominal impedance, which is usually based on a mid-range frequency such as 1 kHz.  That would also add to the concern.  Again, though, these concerns just relate to sonics, not to any possibility of damage.

If you'll let us know the specific model of your CJ preamp, and whether the modifications that were made to it included an upgrade of its output coupling capacitors (especially if it is a tube-based model), and the lengths of the cables that would connect it to the power amps and to the sub, I can comment further on this.

Regards,
-- Al 
 

Al:  Thanks again for your help.
The preamp is a vintage CJ P11 tube model.
i don't really know what modifications were done except that I sent it out not working and it came back sounding better than ever. I don't know what they did, if anything, to change the design.
My connecting cables are about two feet in length.