@stuartk I have front ported speakers. I’ve learned that the physics of bass is so room-dependent, that if pulling them out as far as you have doesn’t address the issue, the location of the speakers ports won’t matter. Bass radiates.
Listening position and speaker placement are the biggest levers. I solved my bass issues with a multi-front assault -- multiple subs, shift of positioning, and treatments. Given your specific situation -- and I’m heeding all your constraints -- what I would do is measure the room using a mic to see where the bass humps and nulls are, and then positioning things to try to mitigate. Keep in mind that various other things in the room which can be moved or shifted might change some of the bass response. A bit. The physics of bass is really quite obstinate.
A diagram or description of your room and dimensions would help.
Listening position and speaker placement are the biggest levers. I solved my bass issues with a multi-front assault -- multiple subs, shift of positioning, and treatments. Given your specific situation -- and I’m heeding all your constraints -- what I would do is measure the room using a mic to see where the bass humps and nulls are, and then positioning things to try to mitigate. Keep in mind that various other things in the room which can be moved or shifted might change some of the bass response. A bit. The physics of bass is really quite obstinate.
A diagram or description of your room and dimensions would help.