I would get the Paradigm signature sub(s).
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The "blend better with my SIGS" issue is a matter of setup. So changing subs will not solve that problem. It's true the M&K will not play as cleanly loud or low as some subs on the market today.
Paradigm Sig Servo
JL Audio F113
Considering the cost, the SVS is an amazing value. It was reviewed in the last Stereophile.
To address the blending issue pay more attention to setup (level matching in particular), consider a Velodyne sub (say a DD-15), use a Velodyne SMS-1, or use an AV receiver with good room correction.