Make sure your highly modified Stereo 70 does not include a solid state rectifier with a lot of filter capacitance. Unless there is some kind of ramp up circuit, that's a sure way to damage the secondary side of the power transformer in a hurry. I already have one on the floor of my office where someone 'modified' their amp only to short out the winding of the transformer. Regarding tubes, I would try a different EL34. The EH's might be the problem. Just understand that if the tubes, or the updates are new, that will take time to break in. Normally output tubes can take 20-30 hrs. to break in. That does not mean to leave the amp on 20-30 hrs. Just keep listening to it when you can, and the sound will slowly change as things break in.
Tube replacement for Dynaco ST 70
I just purchased a highly modified Dynaco St 70 and it came with EH EL 34 and I would lke to replace them since I feel they are a liitle thin and veiled on the upper midrange and lower treble regions where the voices are produced. Any suggestions on replaceing them with tubes that will open up and unveil the voices. I can uses 6L6 or KT 66.