OK, so I have been thinking about it this week. I remember Glen has used felt inside the 4/12 box, in front of the speaker, to calm the trebles down on new Celestion speakers. I tried this also and I didn't like it that much. Hm. But I also recalled visiting a car detail shop here in Portland, Tommy's in SE, and ran into Tommy spraying some wheels with a black rubber spray paint. He told me this stuff was a great product and easy to apply.
Fast forward to this afternoon, I went to Home Depot and bought a can of it (black), along with a piece of aluminum decorative trim 12"X24". My Marshall cab is a 1960A model with the newer very plain black cloth grill. First I gently pulled my logo off. Next I pulled my speakers out, placed my cab on it's back, set this aluminum trim piece on top on my cab (the outside of the cab) against the grill cloth. (You have to bend the aluminum piece slightly to accommodate the slant cab but that's easy.) You can only do one side at a time, so I taped off one side and around the cab border and began spraying rubber paint. Then I did the other 1/2 of the cab.
Now, in hindsight, I could see where this particular cloth could probably handle being painted all over, with no 'plain' unpainted cloth remaining, but I wanted to somewhat hang in there with the idea of the Collins' cloth, i.e., gaps of untreated cloth next to the rubber application. In the case of the Collins' cloth, the rubber 'strips' do not allow for sound passage - such that the speakers are actually being heard via the very thin gaps between the rubber. Technically this is knocking back some sound, how many dbs I have no idea, but also dampening the treble frequencies. In my case, sound can still pass around the rubber in the untreated areas, but also directly through the rubber treated area as well. Which is to say it does not exactly mimic the properties of the Collins cloth. Actually it is quite a bit removed from it, ha. HOWEVER, the rubber applied to the cloth does have an impact on the sound I think. In the case of the Collins' cloth, people report that the treble frequencies are reduced and that this is happening because the rubber is dampening these frequencies. Eh, I don't have near as much rubber (the large rubber surface area on the Collins' cloth serving as a sound block) on my cloth as his, but still, I think the rubber serves as some sort of treble dampening. I'm happy with the results.
Follow up:
I thought about the cloth treatment that I had done and decided I needed to be a bit more radical if I was to approach the Collins cloth affect. So I pulled each speaker and sprayed the rubber material on the entire speaker area from inside the cab. This particular black cloth is fairly open and after this added application you can still see daylight looking out from inside the cab - but less than before. I don't know how to express this maybe, but at this point, as far as the speaker audio is concerned, it is passing through a grill cloth made of rubber strands.
Also, I can see where continued application would start to close off some of the smaller openings in the cloth. Which might begin to emulate better the Collins cloth, but that's for another day. Play test tomorrow.