I recently dealt with this problem as well and unfortunately there is no easy solution if you are unable to move things around. The cheapest and best solution is to move the speakers out from the wall behind them and/or move the listening position forward/back in the room until you have an even response. In my room having the speakers 2' vs 2.5' off the wall made the difference.
I have also tried room treatment using 4" acoustic bass trap panels in combo with bass trap wedge foam. IME minor room treatment like this helps a little but in the end will not cut it unless you are willing to go all out.
Finally, other things you can try other than changing speakers are port stuffing (probably wouldn't work too well on your Thiels) or some sort of parametric EQ. If you have a highly adjustable subwoofer you can also try to adjust the phase, polarity, and cutoff such that it cancels out the offending boom from your speakers without affecting the other frequencies too much. Good luck!
I have also tried room treatment using 4" acoustic bass trap panels in combo with bass trap wedge foam. IME minor room treatment like this helps a little but in the end will not cut it unless you are willing to go all out.
Finally, other things you can try other than changing speakers are port stuffing (probably wouldn't work too well on your Thiels) or some sort of parametric EQ. If you have a highly adjustable subwoofer you can also try to adjust the phase, polarity, and cutoff such that it cancels out the offending boom from your speakers without affecting the other frequencies too much. Good luck!