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.