In my efforts nothing passive actually works. If it does work it's more miracle than anything else, see, the problem with using so much passive absorption is that it's not/never linear through the frequencies, so you'll likely remove valuable musical tone before you effect a solution. The other solution is membrane traps, Helmholtz resonators and diaphragmatic units all emit noise of their own, and every one I tried that noise was audible and sucked.
Barry, right above, seems to have found a good solution, albeit an expensive one. What Barry found is very well hidden from the world... I've never found it before and I've looked pretty hard for active solutions ever since I 'invented' one myself. You see, I was looking to see if I could sell my 'invention', until I found the pro audio world already knew this solution...
Essentially, subwoofers with inverted phase placed after the listening position with the right amount of delay will do what or close to what Barry's PSI Audio AVAA Active traps do. The more the subs have the sonic signature of the mains the better. With either solution
I made the setup with two unused cabinets I had and some cheap woofers and a miniDSPHD. So my out of pocket, because I had 2 plate amps and 2 cabinets, was under $300.
It takes more messing around to get my budget solution dialed in than the PSI Audio AVAA Active traps, but with a savings of $3700/$7700 I'm cool with that, especially as I designed and built a whole new separate epic system for under that $3700....