Arthur, with regard to the possible choice of a Hsu sub, a point is being overlooked: the internal low-pass filter which has a maximum setting of 90Hz doesn't have to be actually used. It can be set to bypass and the inherent response of the sub to over 100Hz would then be in effect, to be rolled off appropriately by the 100Hz crossover in your receiver so that it blends well with your M22s. A suitable choice well within your budget would be the STF-2.

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Enjoy the music, not the equipment.