How are you planning to configure your sub? I am not aware of any problem with large fronts and the sub set to "on," but there has been some discussion of problems with the "L/R+Sub" setting and the way the crossover setting is applied (a problem that seems to be related in at least some cases with "rolling over" from 200Hz back to 40Hz). Unfortunately I haven't had a chance to do any testing in my system to try to get a firm understanding of what modes may or may not cause problems, so I'm basing my response on some of the comments I've been reading lately.