Mods for DYNA ST-70, What do you think??

I agree with Sean's good advice. Also,the ST-70 could definitely benefit from power supply upgrades especially in the filter caps, for increased headroom. Doubling the microfarad amount is not uncommon. Frank Van Alstine has been doing excellent mods/rebuilds on these old Dynaco's for years. He would be the first place I would check. He used to offer complete mod packages, in stages, for reasonable money. Probably still does. He's an old timer and he knows what he's doing, especially with Dynaco.I think his company name is Audio by Van Alstine.

Leave it Stock.Only upgrade the passive parts.

There's a guy that has done MAJOR re-design of the Dyna PAS-2 and PAS-3 using very advanced computer simulation and a lot of know how. His name is Norman Koren and he's had more than a few articles published in various tube / tweaker mags.

I intend to modify my Dyna pre using his basic suggestions once i get the time. Don't know if he's done anything with the amps, but you can try checking his website. It would at least give you an idea as to what is involved in modifying / upgrading a unit of that nature.

AVA ( Audio by Van Alstine ) may be another good source for parts and schematics but i don't think that his ideas are near as progressive. Sean
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http://www.normankoren.com/Audio/

I think soundvalves.com points out that even the stock version has some easily correctable mistakes that to do just a parts upgrade would be a bit of a shame. So at the very least/minimum go their route, along with the better parts. Now if you want to take it further others have presented those options too.

If you do nothing else get rid of the selineum rectifier. Replace it with a diode. These things blow up and take other stuff with them and when(not if) they blow they emitt a toxic cloud into your room!