Doesn't mean you couldn't be even happier if you eliminated the microphonics though!
You should be able to pick up a decent mil spec 6080 or 2 on ebay for <=$20. Not as sexy to look at as the curvy glass 6as7g's, but they are generally nice quiet tubes designed to handle some abuse and vibrations. Search for "jan 6080" - and there are several different lots akin to: http://www.ebay.com/itm/Tube-2ea-6080-RCA-1ea-JAN-6080WB-Sylvania-tstd-amp-radio-amplifier-ham-/390746501948?hash=item5afa4e733c
I don't know. I know some who prefer a slightly microphonic tube. Theory is that it has an ever so slight widening/echoing effect.