Well, the JCM900s are blamed for their diodes in the pre-amp.
Total rubbish, they can sound great. The problem is only in peoples heads: If-its-not-all-tube-it-cant-be-good. Bla bla.

However, the 900s are a very nice thing as "tuning objects". A skilled amp tech dude can tweak them to *really* good sounds.
I once had one, but sold it. Today I use a (tuned) 6100LM for guitar playing. Best heads Marshall ever made