I am powering my 25-31 cs+ with an Adcom 555II bridged and have had zero problems. I sent Adcom an e-mail just to check before I hooked it up and they replied back that its safe to bridge it into a 4 ohm load. Another note is that according to SVS the subs stay above 4ohms and almost always stay above 6 ohms. Hearing it from both sources made me feel perfectly comfortable bridging my 555II and I haven't looked back.
One thing to keep in mind about power to a CS-Plus is that the sweet spot is right around 350 to 550 watts. Obviously the use of a subsonic filter (or not) is just one of the many variables here.
You could have a kilowatt hooked up and not have a problem as long as the sub was properly set up in a typical room (the extra amp power would just never be used) so bridging or not bridging shouldn't bother you.
In the end, you will get FAR more utility (say for those large rooms) if you added a second CS-Plus as opposed to jumping from 400 watts to 800 watts powering one. Based on what is mentioned above about this amp's power I'd probably run it not bridged. Trying it both ways is always advised, just make sure you use a SPL meter and a test disk to calibrate. It wouldn't be hard to smoke a woofer with 900 watts (and no subsonic filter) that was calibrated "by ear".
Ron, I think I will run the sub with the amp in stereo, this seems to be the safer way for both the amp and the sub. I do have an SPL meter, I wouldn't calibrate my home-theater by ear. I have a big room but not huge, it's a living room that gives way to the kitchen but I don't listen to movies or music at extreme levels, I take care of my equipment and 38 year old ears.
Thanks, Brian, I'm still waiting for someone to send me Tumult...perhaps.
Luis, the '555 was of the best bass amp I've ever owned, and I regret selling mine in the late 80's. Just run one channel, you won't need all the extra power, and you have a whole other channel for another sub if needed.