here's what I wrote back in 1989 when I compared every available ST70 mod . . .
Sine wave frequency bandwidth was from 5 Hz to 6 KHz, not much better than the stock Dyna. Square waves remained fairly square from about 60 Hz to 3 KHz.
This mod definitely changed the sound of the amplifier from being warm and rounded, to being more neutral. The music came thru very clean and detailed without being harsh.
The sound was open without the veiling noted above, although the soundstage seemed to be more recessed. Cymbals and bells were reproduced with a clean airy sound around them. The bass reproduction was about the same as the stock Dyna, probably limited by the power supply. There was a noticeable lack of impact with too much smoothness.
The dynamics of the music was better but still not as good as the three mods below.
(VTA, Curcio, GSI)
Here's a link to the original article . . . http://www.tubes4hifi.com/st70mods.htm