Yeah, I dunno... I think I'll stick with OT shunt, or Cathode interrupt, no guesswork on my end. There is nothing in a guitar amp that needs that kind of accuracy.
I have one. Semi-expensive, but there's a pretty cool convenience factor in having all that info available in one display. Plate voltage, bias current, actual tube wattage displacement, and with 1 or 2 power tubes.
This is exactly something I would buy. I have a Eurotubes Pro-One bias meter and their basic probe intended for use with a multimeter. The meter is nice and convenient; I like being able to see the actual plate voltage in real time. But having two of them in the same housing, with power calculations, would be sweet.