Looks like you have tried all of the things suggested about using sub's. As I did. The only sub set up's that ever worked for me were rolling the main speakers off much higher than 40 hz and placing a single sub between the speakers. Elegant and simple. Maybe not the best sound in an absolute sense however. Next was using a sub for reinforcement below 40 hz was crossing the sub off below 60hz (you would make this decision judged by the crossover slopes) and running the main speakers full range. That had the better sound and the woofer placement was controled by the actual bass FR (flat or specific FR boost, depending on what you want to get.
FWIW, ultimately I found that simplicity of set up (i.e. optomized main 2 channel two way speaker set up) was, for me, going to be superior in the long run UNLESS there are nulls and nodes (at the listening position) which are created by the set up and one need subs to help flatten the FR. 2 subs are usually more effective, at least that gives you the opportunity to be more effective by placing them where ever they work because they are asymetrical to each other, the main speakers and room dimensions. They don't have to be symetrical. And in my experience being able to play with the phase relationships can be a good thing if your crossovers allow.
But for me in the long run that was all too fussy and kept my attention too much on acoustics and not so much on the music. My subs are in the attic and I use floor standers +/- 3db to 28hz. When there IS deep bass in the recording I get it. What I don't get is bloat or boom and I've found that I prefer that. One less thing to think about.
BTW, less you want to just give up, setting up a system with or without subs is NOT a walk in the park. It can take an average audiophile years to really optomize a system set up. There is so, so much to learn and it really does take a lot of listening time to do it right. Good luck.
