Subwoofer - is there one that would work in this situation?

Mr. Chesky's -6dB minimally controlled sub-bass speakers are extremely forgiving of room placement simply because they don't output frequency low enough to excite a rooms bass modes.

A properly positioned and optimized -3dB subwoofer can provide the delicate subsonic presentation for almost half the price. Some offer remote dsp control and a few customizable presets to compensate for Cheskey's or your own personal taste. Way more fun.

IMO, you’ll be wasting your time and money adding a subwoofer to those beautiful Janzens. Those speakers go down to 30hz +-3db. Adding a sub might ruin what you have!

I was wondering if that would be the case

I just read the measurements in Stereophile. This is a speaker which you want to put a DSP crossover in front of your amp. You need to tame it’s peaky behavior around 70 Hz and the location of your sub will also cause you some further issues.

A DSP crossover, set to HP at 80 Hz And smooth out the bass peak + the appropriate EQ for the subwoofer is going to be your best bet. Of course, it’s a difficult location. Try the AM Acoustics room mode simulator to give you the best places to put your speakers and sub.

I’d forego doing this at all unless you put an EQ in front of your amp.  that peak will make it much harder to integrate well with a sub, but.... and this is key, rooms turn all bass specs into a hot mess, so measurement first is going to save you.

Kef KC62 might be just what you're looking for.