I had Harbeth SHL5+ with 2 RELT9i and the sound was very big and punchy. I am well versed in technical subwoofer integration and ultimately the best I could do still had compromises. A subtle but noticeable slowing down of the micro dynamics and transient speed of the main speakers, easily heard when the subs were switched out. An occasional overbearing bass despite perfect level matching, basically either too loud or not loud enough as if the levels could not be finely enough tuned. Granted I am a very particular listener and you might find these issues a compromise worth having. You may be best served with an amplifier that can grip the 40s so that they can deliver the bass by themselves which they are capable of. A Rotel Michi for example. Â
My final answer was a pair of full range highly efficient british towers with special flared bottom port for sub bass and they do the job well. Â