Interesting system. I'd agree with Aball, except the kind of buzz he's talking about usually remains pretty much constant regardless of volume setting -- especially since you have a ss preamp. (A nice combo with the Mesa BTW) So you may have some very low level tube buzz, but there may be more coming frome something else. I'd need to know more about how your system is hooked up. (It's unlikely, but maybe the exploded crossover networks dribbling across your floor are picking up some RFI, ha, ha!)
1.) You shouldn't need a regenerator if you have ded. circuits AND your utility company supplies reasonably uniform power, both cycle and voltage. Uniform voltage is especially important for tube amps, unless they have auto-biasing.
2.) Always UNPLUG the system when thunderstorms approach. Just tripping the breakers doesn't always work, and NO regenerator or (ugh!) conditioner, can protect you.
3.) Since you are running digital gear, it could be putting digital "hash" back into the power line and into your preamp -- even when it's turned 'off') Try unplugging the 850 and see if it helps. And in any case, make sure you are uning a quality PC with a floating shield on your 850, AND
3a.) You might want to consider a balanced power unit like the ExactPower SP-15A or one of the BPT units for your front end stuff (and they have digitally filtered outlets built-in for the 850) That would be the finishing touch IMO -- but I don't really think you need a regenerator when you already have ded. circuits.
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