This amp is a black sheep in the world of British amplifiers. They are descendants of those legendary 70's Hiwatts, but they are different in ways that have earned them a reputation for being noisy and hard to control.
These amps can definitely be noisy. But why? Several reasons. First, they have an "active" equalizer. It uses four 12AX7s to amplify each frequency band, and that introduces some hum. Second, it has this "sensitivity" switch that boosts the voltage on one of the preamp tubes, and that makes everything run way hotter (and way noisier). Third, people mod the crap out of these amps and do a terrible job. Finally, this amp has huge Partridge transformers that pick up radio stations like no one's business. You can overcome all of these problems. Use good tubes, don't use the sensitivity switch, get it serviced by a good tech (which you should've done anyway), and be careful where you put the amp in a room. Good? Good!
I love this amp. Those huge transformers make for a really tight, hard-hitting sound… even in the low bass frequencies. The output stage stays pretty clean at high volumes, so it's a great canvas for pedals. The preamp also overdrives well: if you drive it hard with a boost pedal, you get a touch-sensitive distortion. The active EQ is warm and versatile, and so you can get so many sounds from it. I was able to dial in my favorite sparkly Fender-style tones from it pretty easily. All in all, it's a great (but under-appreciated) amp.