Felt over some holes on the bass side certainly helps reduce overall volume.*
A slightly different effort to use felt on a single block of bass side reeds to reduce volume very slightly but brightness a bit more with a felt strip placed under the block actually worked quite well.
The highest set of reeds on a "Super Salas" one of the many models produced by FIAS ( probably in the forties-fifties- this one being double tone chamber 4/5 model) had no provision in the two LH switches to silence it (no slider in the foundation plate for this block). It was quite strident and I found it unpleasant.
When I first acquired the instrument, some prior owner had actually placed a strip of tape over the holes in the bass of the block- thus silencing it. It presumably had annoyed them as well.
I removed the tape but, as above- it irritated me on much music.
I tried a thin strip under the block with 8mm holes in it to reduce air flow- one under each opening- but it had surprisingly little effect on the relative perceived volume (I suppose the restriction of the reed itself was actually close to than the flow restriction of the hole in the chamber). And at other than moderate to loud volumes its restricting action on the airflow made the reeds run flat at the lower volume levels- only a slight mussette effect but again sort of irritating.
I tried another path and placed an uncut strip (uncut as in properly sized to lie neatly under the block on the foundation plate but with no openings) of moderately firm felt under the block so that the air could pass through it (the felt being quite open to the passage of air through the weave), but the reverberations of the reeds in that block would be dampened by the felt at the bottom of the chambers.
Surprisingly enough, this worked quite well. The volume itself for the high set of reeds is not much affected but it really is less bright the overall balance of the bass is substantially better. Not dark and shaded, but no longer in your face. The flat at low volumes problem also is not present.
Putting the felt in required cutting away a strip of the sueded leather from the foundation plate and shaving a bit from the end of the block so that it fit snugly, but not tightly, under the hold downs for the block. I tacked the felt down with some adhesive to keep it in place, as the sueded leather I removed had been. Had I left the sueded leather in addition to the felt the block would have required a lot of shaving to fit under the hold downs. Lateral air leakage through the felt has not been an issue. It would only occur when a note was being played and the resistance to leakage though the compressed felt under the block about the open hole is so much greater than the open path rhrough the reed plate/toungue that it's just not an issue at all.
A specific instance on a specific problem on a specific instrument. I'm not sure that applying it willy nilly would be a good idea unless you've a real similar issue and more than a little patience. Reversing it is straightforward, but would be a bit of a pain.
Apologies for typos/howlers. Henry
* I assume you <DeBra> were speaking of external case holes in the bass die of the accordion vice the holes covered by pallets on the bass side.