Felt diffraction reduction advise

I have a question. If the creator of the speaker thought it needed felt, wouldn’t he or she have included it? It’s cheap and easy to apply. Or would it be added to change the highs in a unique situation?

Of course, and some do, some don't. I think this is partially a matter of aesthetics, but also whether the designer even thought to experiment with it. I don't think felt will always be desirable, but I do think it's a simple experiment you can do.
It's no different in my mind than playing with cables or toe-in.

Excessively deep foam/felt though becomes something else. Then we get into waveguide like behavior. If you are taking that approach you are approaching speaker redisign so might as well start from scratch.

I think Duntech or Dunlavy's used this deep felt approach as well.

Best,
E

Vandersteen's is deep enough to act like a bit of a waveguide.

And yes, it does help. The felt seems to reduce re-radiation directly around the tweeter enhancing the speaker's disappearing act.

Even craft felt works, but Madisound also sells some pre-cut.