HT and Stereo no Passthrough

Hi so I integrated my stereo into my home theatre setup but my pre does not have an ht passthrough.  To bypass this I just dial the volume on my pre to 12 o'clock and have the ht system calibrated with that volume on the pre. Everything works and I have no issues with it so that seems to have worked fine; however, i now notice that I might have a problem, since doing that I have went through the pre tubes in 3-4 months. Does anyone else have any suggestions, changing the pre is not an option as I really love it.

I use a line level audio A/B switch to select stereo preamp or AV prepro.  Goldpoint make some high quality balanced and single ended selector.