It probably depends on what you mean by low volumes...I generally have my Master Volume around 9 o clock to 9:30...much lower than that and it does sound buzzy...you kind of lose the lower end of things., and all Mesa's have a sort of fizzy top end that's more prominent at those low volumes. But at 9 o clock or higher I get great, full bodied tones high gain tones.
On to your question....with another long reply...
An amplifier can be divided into pre-amp and power amp stages. The pre-amp stage is the primary tone shaping stage. It's where your tone stacks are located (with most modern amps these are the last part of the preamp stage). And the 12ax7 tubes that are part of the preamp stage can be driven to break up (controlled by the "Gain"). The higher you turn the gain, the harder you drive the pre-amp tubes and the more distorted the signal becomes.
This signal is then fed into the power amp stage, which is controlled by the "Master". The higher you turn the Master, the harder you drive the power tubes. At higher settings the tubes start distorting providing additional distortion to the pre-amp signal. At lower settings, the power amp amplifies the pre-amp signal, but does not add any distortion to the signal.
So, with the Gain high and the Master low, what you're hearing is pre-amp distortion. With the Gain low and the Master high, what you're hearing is power amp distortion. When both are high, you are obviously hearing both.
The general consensus is that power amp distortion sounds better...that's primarily what you hear on classic rock recordings of the 60s and 70s. But there are practical problems with that...for one, most people can't stand that much volume for very long. Also, you can only get so much distortion out of your power tubes.
Enter pre-amp distortion...now you can get distortion at reasonable volumes (though many say it dpesn't sound as good as power amp distortion). Also, it's the only way to get higher levels of distortion (short of digital or solid state distortion). Of course, most modern high gain tube amps also employ diode clipping to get high gain sounds, since the pre-amp tubes can only provide so much distortion themselves. Marshall first incorporated diode clipping in it's JCM800 series and has used it ever since. I don't know for sure, but I'm guessing it's also incorporated in most of Mesa's designs (does anyone know for sure?)
Anyway, I feel that Mesa does pre-amp distortion better than many amp makers, and that I can get great tones even at reasonable volumes.
