I feel like I could share my own limited experience in this thread, as I went through a similar search as the OP a few years ago, trying to find an attenuator to play on the sweet spot of a SVT VR at any volume I needed, lower than the full blast of a cranked SVT.
Following that idea, I learned a little about how attenuators work, what they can do and what they can't, depending on the design, for guitar and for bass tube amps.
I now do own a Weber 200W Mass attenuator too, that I do occasionnaly use with my V4B, but knowing what to expect from it, and what not. It seems there was a reason late Ted Weber developped an attenuator incorporating a speaker motor to "emulate the reactance effect that a speaker would provide". A simple dummy load does not mimic the way a speaker motor constantly interacts with the amp, and the effect it has on the tone. Maybe I couldn't tell the difference on a blind test, between an amp attenuated by a simple dummy load and by a Mass-like attenuator. I haven't done the test. But I feel like I understand and trust the physics of it, enough to believe that it would make a difference to me.
As for the SVT, my conclusion was that it is designed to be loud. It can be a little quiet and clean, but to my ears it really get its tone when loud, and of course when reaching the sweet spot. I could play on the efficiency and number of speakers to be a little quieter or a little louder, but basically I felt like it was just not logical to try to get it to be a quiet amp by attenuating it to a point where it was loosing its tone and guts. I thought it would be like using the wrong tool to get the job done.
So I found myself a V4B, and a B25B, and I'm very satisfied with that choice. I use the Weber Mass when I want to record the V4B pushed to overdrive, in a small room, or loose a few dB while staying on the sweet spot.
I don't know if this is useful, but from my experience, I thought that the OP got the best possible advice in the first ten answers he got.
