PROAC D48R Bass Booming

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.  

The first step is always proper speaker placement.  It is also one of the hardest and most time consuming pursuit.  It is not enough to apply the rough generalization that--away from the corner, bass is reduced.  You have to move the speaker inch by inch to find the right spot.  Google the Sumiko method of speaker placement. 

Room treatments might help, but, that is a step that should come after speaker and listener location changes have been tried.  With excessive bass, look to employing bass traps in the corners of the room.  Traps tend to be big, but, that size is required to tame the long wavelengths of bass sound-waves.

ProAc D48s do tend to have a full, and not particularly tight, bass response.  You can alter that a bit by, as someone above mentioned, adding some resistance to the port output.  Aside from this "modification," any other equipment change will have only modest effect on the sound.  You can try leaner sounding speaker cables and interconnects.  I don't know about the particular amps you are considering, so my general comment is that it is really hard to predict how any given amp and speaker, in any given situation, will play together; you have to try out the combination or count on luck.

gnanasekhar,

One more thing to consider--if the floor in your listening room is a suspended wooden floor, the floor could be acting as a giant sounding board which could be at least part of the problem.  With such flooring, the speaker should not be coupled to the floor (e.g., by using spiked cones), and instead, isolating footers or platforms should be used; you want something that will absorb/damp the bass energy that is shaking your speakers rather than coupling that energy to the floor.