[Isaacc7]

These impressions seem to mostly hold up regardless of which output tubes I use.

GE 6sl7
Sylvania 7f7 "chrome dome"

I found both of these to shift the emphasis up in frequency. While the 7f7 had remarkable bass control and extension ultimately I didn't enjoy the sound signature. It just gated on me and I found it fatiguing.

Sylvania 7af7. This is an odd tube not officially supported but I have heard of it substituting for 6sn7 tubes in other amps. It's gain is between the 6sn7 and the 6bx7. It's a round plate Sylvania tube so what the hell, I gave it a shot. I don't know if I just have a bad tube or if the 7af7 just isn't compatible but it did not sound good. Closed in, collapsed sound stage, grainy. Just bad.

My favorite input tubes have been various 7n7 tubes. As far as I can tell they are all Sylvania regardless of the label. Old Sylvania 6sn7 tubes are all well regarded and so I don't think you can go wrong. My favorite one so far is a tall bottle with a short silver getter on the top. From the 40s, it is open, balanced, and with impressive bass.

I have a slew of other conventional Sylvania 6sn7 and 6sl7 of various vintages that I plan on trying too. I'm actually hoping my 7n7 stash ends up being just as good. Then I can sell the regular tubes lol.