James,
Interesting topic! One thing we need to remember is that it's not only the hammer felt that changes with humidity, and the other changes will affect the tone as well. It's sometimes hard to separate out what has changed the sound; hammer felt, hammer centers, regulation changes, or even keyframe bedding, and of course bridge /soundboard growing and the resulting solidifying of the tone.
Also, hammers are different in how they react. A hammer that has been needled a lot will act different than one that has not been needled much, and a hammer that has been lacquered (or otherwise hardened) will react differently than the other two.
In my experience voicing concert grands that have moved from place to place and changed from dry to wet to dry to wet . . . (etc.) Kawai hammers (which are needled a lot in preparation) will soften with high humidity and sound warmer. A little ironing will bring that right back, as it drives some of the humidity away. The few times I have seen Kawai hammers exposed to fairly long VERY high humidity they have fluffed up and become very dull in tone. I filed with fine sandpaper (1,000 grit strips, pulled fast with a lot of pressure) to bring the tone back up.
In the Steinway concert grands I used to service I felt they got stronger toned (better power, not so much brighter or softer) when humid, but I think this was more the action changing instead of the hammers. I had the feeling that the hammer felt fibers were more locked in place by the lacquer.
I have had a few sets of Abel and Ronsen hammers in older Kawai grands that were not needled much, and with those I also felt the tone got more round and full sounding in high humidity. So those are where I surmised that the un-needled felt was trying to swell but the unbroken fibers from lack of needling increased the tension and spring in the felt. I never heard any get brighter, though.
Long term humidity does sometimes change wool felt in a way that stays. An example is action centers that worked fine in normal but variable conditions for years, then the room gets flooded or something and they freeze up permanently. Heating them and drying them doesn't fix it – the felt has swollen and stayed there, so they need to be broached and repinned. This is just an example – I think the same is true in hammers as I mentioned above, but different hammers might show it in different ways.
In Yamaha grands the hammer centers tend to be VERY free normally, and I would think your sample pianos might have gotten "brighter" from the hammer centers becoming more tight and firm. I haven't worked so much on Yamaha hammers, but I could also see the wool changing in a way that increases power, but I'd be surprised if it was actually "brighter" as your customers thought.
Finally, I once did an experiment with an un-needled Kawai GS grand hammer (from the 80s, very hard). I submerged it in water for over a week! I listened to it before, and it was bright and nasty in the unvoiced state. After it soaked I let it dry out for a month (or longer, I'm not sure), then put it in a piano. It sounded a little mellow but was not bad! After filing the bulbous swollen shape back to normal, then fitting it to the strings and such, it really was a very warm but full and powerful tone. Certainly NOT a recommended voicing procedure, but with this un-needled new hammer it wasn't bad, and it illustrated how that particular un-needled and super hard pressed felt was changed permanently by the extreme moisture.
Don Mannino RPT