To expand on what SG said, they are known for loud AND clean. Still a little woody with EL34's but they were all but done by the time Marshall was master voluming and all but gone by the jcm800 years. Other than the Pete Townsend fans it was not considered the "sound" of classic rock. Some people say they are too bright, abrasively so. Others say BS on that. Some say the equivalent Marshall has more tonal capability. Others say BS on that too!Most say they are super pedal friendly. Most also say the KT88 versions sounded best - which I can certainly relate to. Some bass players swear by them. One of note was Glenn Hughes who used 400's in the Purple years. Although I heard from Ritchie's amp tech that the open frame partridge transformers howled like crazy when the amp was turned up over 6 or 7. I played a 50watt version years ago when I was asked to come up and play with a band in a live situation. I thought it sounded really good and was loud and bold, but it was very pedal driven. I like a cleanish transparent amp, which I thought it was. I think there are enthusiasts out there but, like SG said, people seem more concerned with producing less watts than even 50 these days, much less 100, 200, or gosh forbid 400! Original construction is still unmatched. Cabinets are still unmatched.
Jim