Subwoofer for classical music listener

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.

OK. The manual for the PV11 indicates a nominal output impedance of "less than 500 ohms" (presumably at mid-range frequencies), and recommends a minimum load impedance of 10K ohms. Given that output impedance and the short length of the cables to the amps the added capacitance of line-level interconnects that would connect the preamp to a sub won’t be an issue, for any reasonable length. But I would look for a sub providing a line-level input impedance of at least 20K, as the parallel combination of 20K and the 22K input impedance of your amps results in an overall load impedance that would be seen by the preamp of:

(22 x 20)/(22 + 20) = 10.5K.

Unfortunately the specs I can recall for many of the REL models indicate line-level input impedances of 10K, which with the 22K input impedance of the amps would result in a combined load impedance of only 6.9K, well below CJ’s 10K recommended minimum for the PV11. The SVS SB-1000 that was suggested has a line-level input impedance of 47K, which would be great. Some of the relatively expensive JL Audio models have unbalanced line-level input impedances of 50K, although if I recall correctly the input impedance of some of their other models may be too low to be acceptable. You may have to contact some of the other manufacturers that have been suggested to determine the input impedance, if it isn't indicated at their website or in the manuals that are provided online.

Good luck. Regards,
-- Al