Dynaco equipment was designed to provide decent, credible sound quality at the lowest possible price. As such, the build quality and cosmetic are sacrificed as are features. McIntosh is designed with the opposite approach of providing best sound, best build quality, most features and so a much higher price.
The gain controls on the MC30s are a nice-to-have feature. That the ST 70 does not have this feature is one of the ways they kept the price lower. A competent tech could easily install gain controls in an ST 70 if you want the feature added.
Don't overthink the clear-path thing. The gain control is just a variable resistor and is not going to add appreciable noise or distortion to the signal. In fact, the signal path in the MC30 uses four small tubes per channel verses just one small tube per channel in the ST70 and so is a much more complex signal path and yet the MC30 clearly delivers much better sound and lower noise.
