The HE9 is just an M9 with a power regenerator built in. If you already run some sort of power filtering, the upgrade of the HE9 from the M9 would be partially, if not completely, negated.
From early feedback (its been a while, so things may have changed), the power regenerator did have some manufacturing inconsistencies which resulted in a few having to be returned. I personally rather have the power regenerator separate so I don't lose my whole system if a minor component failed. It is really just an eggs-basket thing.
As for the Taurus vs M9, both are amazing HP amps. I'd have both if I had the space, but I chose the M9 due to better speaker pre.