I took off the 6AS7 and left the 6SN7 burning in for almost a week, listened to the amp yesterday for the first time.
I'm really surprised by the stock DV336/6XX combo!
Dead silent, zero hum. The stock 6SN7 sounds pretty good, considering its innards are crooked...
The volume pot as expected on a cheap one, has bad tracking on the lower 1/4 turn, but fine from that onwards... first mod I'll be doing is shunting it with proper resistors for the 6XX.
I used it with my "baseline" reference "system": a 2006 macbook built in audio out, which to my ears sounds really, really good, organic, mid-level detailed sound, not ultra-resolving, but always balanced and engaging, (whose character I know from heart after 12 years listening to it) before I start DAC rolling.
First and most simple "tuning" you can do: find a sweet spot for your audio source output, i.e. while the player (itunes in this example) should be at 100% volume, you'll find that if laptop general volume is at 100%, and DV pot is at 1/4 turn, the sound starts to feel "compressed" and you'll also be on the bad tracking, suboptimal portion of the amp volume pot.
To tune, start first by lowering source volume to say 40%. Raise DV volume pot to 12:00 (6XX), then try output volume at 80-95%, listen to a song you know well, you'll feel music "opens up" at a certain point; use the DV volume pot to adjust final volume and your ears to judge.
Really noticeable with stock chinese tube, if you "drive it hard" compression will be very noticeable (congested sound, lack of separation, etc).
When you get it right, stock tube though definitely being bass shy, is not bass dry.
Pretty decent sounding, though not really "tubey" in character, (more of a hi-fi type and somehow lacking in "glitter" etc) imparts that relaxed, efortless tube character noticeable in things like ambience and decay.
I need a little more listening before start rolling tubes... first one will be russians (6H8C) for sure.
As ecline said, there are really, really nice NOS USA 6SN7 too... look for an old radio repair/antique tube shop, most honest ones will have them used but tested and will provide you % of emission and gm, I can source used, good condition (testing in high 80%-90%) NOS RCA's and Raytheons localy (in Spain!) for around 30€.