If have room, I would just get another sub if you care about the LFE & ULF of your HT. I tried doing what you were doing but somewhere along the chain, I couldn't make the sub work for both configs based on the bass I wanted. If you don't connect the sub to the HT receiver and use Audessy, the sub won't rumble like you expect since Audessy just sees your L/R as large speakers. You'll get bass, but I found it to not be as impactful as the sub connect to the HT receiver.
I opted for the Monolith 12. I dedicated my HSU to my 2ch and glad I went with the Monolith for my HT.
I opted for the Monolith 12. I dedicated my HSU to my 2ch and glad I went with the Monolith for my HT.