PROAC D48R Bass Booming

OP: If you plan to keep these speakers, Find a competent tech and have them add a volume control to the woofer circuit. They could hang the control from a wire so it comes out the bottom (where the bass comes out) so you wouldn't have to damage the cabinet. Problem solved.

Where are the speakers located? How close to a wall? If really close, move speakers out into the room to diminish the bass. If all else fails (assuming these are bi-wire speakers), bi-amp with another integrated amp so you have control over bass volume (I just remembered that my Proac D38s had a funky bi-wire structure. The top speaker-wire posts ONLY FED the tweeter. The bottom posts fed everything else).If bi-wiring practical, the VK-3000 makes it extremely easy with dual "pre out" connections. Another thing to try is plugging the bass port with something (foam rubber, pair-of-socks) anything to reduce the bass. The bass exits the speaker at the bottom so try stuffing some towels in the space to reduce output.

Go to a home store and ask for some pipe insulation. Measure the opening at the bottom of your speaker (looks like four or five inches). They make pipe insulation out of foam rubber and many different sizes so get some big enough to block the bottom of your Proac. This will cost $10 - $15 dollars.