Hi Matt, welcome!
From your description, I don't think that having your mains wired in line with the sub is going to be a good solution. I do not have the Hsu (you lucky dog), but I believe that the crossover is low-pass only. That is, the crossover on the sub is only limiting the high frequency output of the sub, not limiting the low frequency output of the mains. So, you're going to continue to overdrive the mains at certain volumes regardless of the sub crossover setting if you have them wired in line with the sub or if you have them set to large.
I'd be surprised if those Fluance speakers were *flat* down to 38Hz. Are they very large boxes?
So, we're back to having the sub on LFE and the mains set to small.
Does the receiver allow you to set the crossover frequency, or is it set at one specific point? At any rate, if the sub is running off the LFE output, set the crossover on the sub all the way out.
You should try to set up the sub (using a SPL meter if possible) so that you can leave the volume on the sub alone all the time, and just adjust the sub output on your receiver. You may have to continue to make adjustments for music/HT as you see fit, but at least you'll be able to easily quantify and return to the baseline settings on the receiver.
Of course, buying the Axiom Epic 60 Home Theatre system will solve your problem in one fell swoop.
