Haven't yet heard the Descent. I have heard the Stadium III and good as it is, I'd suggest you also add the ACI Titan II to your short list. You could get a pair of Titan IIs for less than a Stadium III. The Titan II produces superbly accurate bass and integrates seamlessly.
Martin-Logan Descent or Rel Stadium Subwoofer
I have heard great things on both subwoofers. I do have ML SL3's for which the Descent is made for(electrostatic speakers). The Rel's can adjust the mid-high frequencies for a seamless blend and are great on all types of designs so I have heard.. I believe the Descent cannot, only tuning for low frequency. Any opinions out there on these two subs considered for my speakers? The price point is about the same. Retail $3000. Thanks, Steve