Is it really the case where the speakers sound great with most recordings, but is too boomy for some? Could it be that it is still a bit boomy with most recordings, but not so troublesome? I ask because any sort of permanent fix to reduce the booming will also affect the sound with recordings that you are not troubled by. It might be the case that an equalizer will be the best approach for you--you can then do recording-specific adjustments.
It may also be the case that a set of subwoofers will work. You can adjust the crossover frequency and thereby cut the levels of bass coming from the ProAcs and then adjust the bass from the subwoofers to your liking.
In any case, your approach of doing more experiments with placement will always be the best first step. But, do not assume that farther from the back or side walls will always reduce the booming bass. There could be points that are actually near the walls that will do the trick; it is a matter of finding points that reduce reinforcing nodes and cancelling nodes. I also generally like bass traps in room corners.
It may also be the case that a set of subwoofers will work. You can adjust the crossover frequency and thereby cut the levels of bass coming from the ProAcs and then adjust the bass from the subwoofers to your liking.
In any case, your approach of doing more experiments with placement will always be the best first step. But, do not assume that farther from the back or side walls will always reduce the booming bass. There could be points that are actually near the walls that will do the trick; it is a matter of finding points that reduce reinforcing nodes and cancelling nodes. I also generally like bass traps in room corners.