honestly depend on headphone for me, if I put on my Beyer T1, I need absolute warmth and bass, I dont think 6sn7 in that configuration is good. Surprisingly 6DE7 is really good for bass, 6sn7 will add punch but it really lean up the bass compare to the 6DE7. 6FD7 is good too but 6sn7 maintain that top end clarity. is it worth the upgrade? depend on what you like and your headphone.
6FD7 add more boom, it doesnt punch as hard compare to my GE 6DE7
If anyone cares to dig through all of this content, takezo wrote a lot about his/her WA6/6SE listening impressions with various tubes several years ago. Type takezo in the search window at the top of any page on head-fi.org and that should take you to takezo's content.
Below are excerpts of takezo's listening impressions:
"6DE7 - Fast transients, good for rock, pop
6EW7 - Softer sound but slightly larger soundstage, good for some jazz, classical
6CY7 - between 6de7 and 6ew7
6DR7 - similar to 6de7 but higher gain
6FD7 - punchy bass, large soundstage, fast transients"
"6sn7 is super linear and offensive to no one's listening
palate but the 6dj8 has a beautiful coloured tone that
is addictive. it also needs the anode plate voltage to be
lowered to around 130v for the 6dj8 and 150v for the
6922 and 7308 to function a long time. it'll work fine at
200v on the wa6SE without any resistor adjustments
but i'm sure it'll lead towards an early grave for the tube."
"favorite ... 6CS7 ... requires
a simple adapter and works fine at 200+ volts. the tone
is as sweet as 6922 but with an even more intimate texture,
soundstage and timbre, especially for acoustic instruments.
acoustic guitar, violin and piano sound great with the 6cs7."
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NOTE: I sent a PM to Deyan and he built 6sn7 and 6cs7 adapters for my WA6SE.