I would consider upgrading the sub cable. I have a Rel B3 and the stock cable is horrible at best. I personally have a Nordost baseline made to Rel specs. Huge difference for the better. Something to consider. I'm not saying to get Nordost but try a better cable.
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Based on these measurements: http://www.avtalk.co.uk/showthread.php?t=10959 distortion doesn't appear to be issue, but group delay might be. The sub is amazingly clean to 25 Hz.
I saw your preamp post... I assume you vetoed the idea of using a pre/pro as a preamp. Something like an Integra DTC-9.8 would handle the bass management without an external controller and allow you to run balanced lines to your A21 amp. Plus, you'd get room correction. Just my opinion, but a "real" preamp is a step backwards.