How hard is this on a tube pre amp?

It's no big deal.preamp tubes last for years. I have my volume control almost at full gain and the tubes are over 6 years old . Preamp tubes are not expensive to replace.

BTW you are better off with the volume control in the upper area on the dial because it is more flexible that way.

Dial it up. No problems. I believe most, if not all, volume controls are actually attenuators and are more linear for stereo tracking (unless they are really fairly extravagantly designed and built) in the upper reaches.

I "slingshotted" pres together from a line stage to a TacT for years and unless you drive it into distortion, so what?
You can do the same thing from a tape deck with volume controls or by setting a phono pre a bit hot.

The volumn control is really an attenuator. You are not hurting anything by turning it up louder. This also will have no bearing on tube life. Enjoy

FWIW, I sometimes push my Ref 5 SE to 80+ clicks out of 104 total clicks when playing some records. Kal said I won't hurt the Ref 5. If you want to be certain, e mail Kal. He's very responsive.