Subwoofer for classical music listener

Now retired, I was a professional classical musician (pianist.)
Having played in many instrumental combinations and performed in and attended countless orchestral concerts, I well know what real instruments sound like.
I simply want to enhance my already quite realistic system with more dimension.
I'm hoping a subwoofer will do this.

Almarg…yeah I saw that bit about the "other" REL inputs but I choose to ignore it so I could ramble on about my RELs…so to revise: In this case, don't be like me.

I am a  50/50 jazz/classical listener, and understand what you mean.  However, if i was in your case, I would not compromise and would have preferred to wait for more money to get myself a better amp or speaker, or both, and no subwoofer at all. This way you spend more for sure, but you get much more benefit for music reproduction, for all music, everyday. 
If you absolutely need the sub and want to keep your actual setup, the best choice and by far for sound per pound, which i personnally know about and amongst all other common names such as SVS, REL, etc... is XTZ. Their Sub 1*12 model is excellent, all necessary tuning possibilites are available, including the ability to play with open or closed vent, depending on personal perferences and/or environment, the bass is extremely firm and controlled, which would be an absolute requisite for classical and especially given the Dalquist nature which lacks bass, but which is fast, even more so with classD amp. The XTZ frequency range is wide, down to 19 and up to 160Hz. The price/quality ratio is near unbeatable, and they sell direct. You dont need a lot of power for sure, with classical you need full control and sound coherence.

rvpiano, happy to hear you've decided on a new product that can be returned or upgraded if not satisfied. Considering the subs fundamental controls and depending on the room and your personal taste you may want to experiment with its location within the room. Since every room has different properties its been my experience that the traditional placement suggestions are just that, suggestions. 

With similarly controlled subwoofers I've had some luck with "the subwoofer at the listening position crawl test" to find the rooms major node then using a pair of inexpensive very long RCA interconnects from Blue Jeans or Monoprice cable to fine tune the subwoofers location. Placing the sub on an office chair or a small moving dolly may aid you in finding its optimum placement.

Good luck, hope you get the lows you're looking for.