have you looked into Paradyme? I’m not from the area, but they list Vintage Stereo Repair as one of their services (along with car audio and home automation/installs). They’re on Fulton Ave. in Sacramento. www.paradyme.com
The ST-70s seem to eat rectifiers with regularity…new market 5AR4s especially. I run vintage 5U4Gs in my VTA (Bob Latino) ST-70 as they seem to have the best reliability.
The suggestion to upgrade the power transformer is a good one if you haven’t done that yet. You mentioned mods, but I’m not sure which ones.
Best of luck with the fix!