high pitch from tweeter

bad/microphonic tube in power amp?
dirty interconnect connector?
Try disconnecting the preamp output, then energize the power amp & see what happens. You'll isolate any posssible preamp issues with that test.

Yes that's very possible. I didn't know the Audioaero Capitole has a tube output. Tubes are prone to ringing, microphonics, high idle noise, etc. especially when showing their age.
Since you're considering a tube replacement, try to chose a good one from among the many options. Kevin at the Upscale Audio dealership has been mentioned here as a good consultant to reference replacement tubes. I know I've read of others too, but Upscale stands out in my memory (I've never dealt with him myself). If you regularly deal with a particular shop, then you might ask them for suggestions as well.
http://www.upscaleaudio.com