I replaced the stock GE tubes with a) 1940's Raytheons, and b) 1960's Amprex "bugle boys" (French DL-92). The Raytheons were not much different than the stock GE's, unfortunately - maybe a bit warmer sounding, but with tons of microphonics and a small, compressed soundstage, just like the GE's. The Amprex tubes are like having a new amp - the highs and lows are much more extended, the soundstage is bigger, and there are very little microphonics! The difference between these and the GE's is huge, and I'm a bit of a skeptic when it comes to tube rolling, differences between capacitors, etc. The Amprex tubes sound so much clearer and more "real" than the smeared, rolled-off GE's - and can be driven harder without sounding congested. The biggest difference may be in the treble, which gives the music much more "air" and provides correct positioning cues, etc. This, along with a lack of microphonics at low volumes, leads to a blacker-sounding background and better detail recovery. These were $25/pair from Tube World, which was money well spent.
Interestingly, there does not seem to be much difference between various 6DN7 tubes run on the S.E.X. amp. Maybe these little 3S4's were subject to bigger tolerances and/or differences between manufacturers?
