Those 4101 JCM900s are infamous for their buzzy, compressed character, and they really sound like a swarm of bees with single coils. PG uses Vintage Moderns which are much better.
Endorsements are done by famed artists to make money. But they tuck their favorite stuff somewhere at home or in the studio, but to avoid contract breeches, they use them discreetly. Would you think John Petrucci does NOT own a single Marshall?