[konut]

I know it is generally frowned on  to put a monitor on a sub. It would simplify greatly my setup if I did so in my small room. Are there any solutions (materials to put between speaker and sub) that would offer isolation and reasonably acceptable SQ?

[Tyson]

Herbies Audio Lab has some excellent isolation devices that work great for this.

[konut]

I have submitted a request asking which product they recommend on their website.

[JLM]

Isolation is not the main objection.  Bass is best generated in a corner or along the wall, mids/highs are best reproduced away from walls/corners.  Perhaps you can try DSP to compensate.  Most isolation products are far too stiff (not compliant for a given weight) to offer isolation.

Best option would be to use a larger, less square, insulated, and dedicated room.  Or simply switch to headphones if your room is so small you can't separate monitors from subwoofers.