Belles 21A standby mode?

I have the 21A also. The standby mode keeps the tubes' filaments heated, so that the warm up time of the tubes is less than 15 minuted vs. an hour or two from a cold start. However, the rest of the tube is not powered, thereby it's life should not be severly impacted. In fact, the opposite may be true; the tubes' life may be extended due to the tubes not facing the harsh turn-on currents every time you power up the preamp. This is also the same as my other preamp, the Audible Illusions L2 preamp.

If you're not going to listen for a while, like a few days or so, I would disconnect power to the pre.

Dave Belles stated that a couple of units went out the door with a missing jumper. Mine could be one of them.
With the 21A powered off or powered on, there's no change in "glow intensity" of the filaments.
I also have a 20A and the chassis above the tubes gets slightly warm but there is no visible glow in any of the 4 tubes.

There is not going to be a change in intensity of the light emitted from the tube filaments. The power to the filaments remains the same on or off. It's the other circuits you change.
1markr, care to comment on the comparison between the L-2 and the Belles 21A sound wise? I also have the L-2 but looked hard at the Belles. I didn't get to demo the 21A. I'm using the AI with a Belles 350A.