No problem!
Connect the sub out on the HK to your retired Sony with an RCA cable. Connect the sub to the Sony's speaker output, using the same (right or left) side that you used from the HK. Sony's volume control sets the relative amount of subwoofer volume, which you would calibrate using the trusty SPL meter. Set the crossover on the sub to where it integrates with your M22s (most likely setting them to small on the HK and match the frequencies). Now mark that volume and leave it there. The HK does all the work, the Sony is just an amplifier.
