I had an amazing sounding 67 SB that was in almost mint condition, and was one of the best Marshalls I've heard. The thing was perfect either loud(with it's own breakup) or at low volume with pedals pushing it. Had it for two years, and around early 2001, I got an attenuator for it, and after about a week, it fried the OT. And it wasn't even pushing it that hard. Amp volume was about 2:00 and attenuator three clicks down. Tried everything from older transformers, new MM, the original rewound twice, but the amp never sounded that great again. So I promised I would never ever use another attenuator again.
Plus, I never heard an amp with an attenuator that sound as good as one without it or with a decent Master Volume.
Matter of fact, I even avoid buying amps from people who uses them with attenuator. With the volume of amps that I've had the chance to own, coincidence or not, the ones that were previously used constantly with an attenuator, were the least inspiring ones, and the ones that got booted not long after.
More power to the ones who likes and use them.
Me? I hate them.
That's my experience with attenuators.