They're not bad, they're touchy, like most silicon used in such an app. If they go, they can short the in/out terminals, which would probably kill the mics, if the TL783C is used for a phantompower PSU. In the Gyraf, the preamp won't die I think.
Unless you wan't to re-design the PSU, just go with the original design. I don't think using a LM317 will be better.
Tube regulators seem to be almost bombproof, at least in my experience.