Just got a few different 12AX7's in the mail from from Amplified parts the other day. Last night, I went through all of them with some headphones on and here's what I found.
-These tubes will probably sound completely different in any other amp. I did something similar with my Orange BT 500 and liked different tubes for different reasons.
-This is a comparison between these tubes only. I did not hook the amp up to a scope or anything.
-I noticed that the differences were very difficult to detect with all controls at 12:00. After setting the controls to the positions I usually use (w/ a cab), differences were apparent (left the same between swaps).
Original JJ ECC83 (Ruby)- Nice lower mid bump, muted highs but a nice Ampegy bark.
JJ ECC803- slightly compressed high end with very smooth breakup when pushed. Big low end with lower mids similar to the ECC83.
China 12AX7- Nasty (in a good way), biting high end and a great jagged breakup. Flat sounding lows and mids.
Tungsol 12AX7- This one was my favorite. It over-all seemed to have this 3 dimensional characteristic. Very sensitive to dynamics and the breakup stays focused even when pushed hard. Nice even high end. No frequencies seemed to stick out so it had the most balanced sound to me.
When you get into clipping territory, it seemed almost like the distortion was in parallell with a clean sound. This gave a great clarity for way I play. This is the one that I left in.
I really liked the China 12AX7 that it came with the best. That or a Sovtek. The China had this ratty character that makes preamp voicing shine. The Sovtek seemed to cut the lows a bit and add some sharp high end (useful for it's darker voicing).
The JJ's either had too little gain (making the amp too clean) or added too much low end. They also made the distortion a bit too smooth.