Tube replacement for Dynaco ST 70

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.

A few years back, I modded a ST-70 and used 6L6GCs for the output. However, I had to replace the bias potentiometer in order to have enough range to accommodate these tubes.

Triode Electronics in Chicago has some some driver upgrade boards available that include excellent instructions for a variety of modification options.

I have the JJ E34L's in mine and have been very happy with them.
I feel that the EH tubes are really good in guitar amps, but prefer the JJ in the Dynaco. notice that these are E 34 L not EL 34- so they draw a touch more current. Mine started sounding pretty good after about 10 hrs. They didn't sound BAD at first , just started to open up after ten hours.
If you want to go the 6l6 route, I would definitely go with KT-66's. The KT designation specifically refers to "Kinkless tetrode" which means that they were designed to remove a "kink" that was present in the frequency range of the 6l6.