Just to add a new twist to your scenario - I was a bit dissatisfied with my Harbeth P3ESR XD with the tiny REL sub so added a REL Classic 98 connected to the amp speaker terminals. Huge difference (after a lot of playing around getting it right).
I was very impressed when I added the SVS 3000 micro sub to the P3ESRs. I needed something fairly discrete (physically) for my living room and it fitted the bill. Setup and control via the app is also very good.
I have adjusted the volume and low pass filter now with the SCM11s though there is still some fiddling to do.