I found the Valhalla amp improved bass performance on my X5s. I was fairly stunned at this given that the woofer was driven by a different amp.
Using a speaker level signal is supposed to preserve much of the sound quality of the source amplifier. REL use this method for their subwoofers and Musical Fidelity offered their Supercharger high power amplifiers that would accept a speaker level signal from low powered amps. This is why you'd notice a difference in bass quality from using different amplifiers.
For me, the active bass in my X3's solved a room problem I have with a large sofa between me and the speakers acting as a massive bass trap. Sit on the sofa and there's bass. Sit at my desk directly behind the sofa, as I usually do, and the bass vanishes, but with the X3s I can correct for my listening position.
