I just completed a not even close to exhaustive comparison of 4 different preamp tubes in my Yorkville XS800, and I thought a few of my TB brethren and sistren might be interested in the results. On test were: the stock Sovtek 12AX7, a GrooveTubes ECC83, an Electro Harmonix 12AX7, and a JJ/Tesla ECC83S. Amp settings were my usual: Bass, Parametric Mid, Scoop and Treble flat. Lo Mid and Hi Mid around 1:30-2:00. The internal compressor was off, and Bass Resonance control was set to Tight. To test the TubeDrive circuit, the Blend and Drive controls were set at 12:00 for all tubes. Bass used was a Lakland 55-02 with various pup selections and EQ flat, and the cabinet was an Avatar CB112 (Delta 12LF) with the tweeter off. After every tube swap, the amp was switched on and left to idle for 5 minutes before testing began. Sovtek 12AX7 Clean: A good, solid, nothing wrong with it tone. A slight increase in volume and warmth when switching from the SS preamp. To be honest, if it wasnt for sites like this, I may never have thought of changing the tube, except for Sovtek 12AX7 TubeDrive: Raspy and dirty (sort of in a good way), but the bottom drops out of the basic tone, making it seem fuzzier than it might really be. Not very responsive to dynamics. Might be OK if combined with an EQ pedal to maintain the bottom. GrooveTubes ECC83 Clean: Very similar to the stock tube, probably because it may be the same tube made in the same factory. I had bought this tube previously to troubleshoot a different amp, so I thought Id throw it into the mix. GrooveTubes ECC83 TubeDrive: See above. Maybe slightly better bass retention than the Sovtek. Maybe. Electro Harmonix 12AX7 Clean: Nice. The amp seemed to have a wider frequency response while at the same time scrubbing the edges off the extreme high end. Those sometimes hard to get to harmonics in the 2nd and 3rd fret area jumped out a little better than Im used to hearing. Did I say nice? I meant very nice. Electro Harmonix 12AX7 TubeDrive: A big improvement over the Sovtek and GT. The high end edge scrubbing carries over here, and the bass and lo mids didnt disappear to the same degree as the Sovtek and GT. You might still need an EQ pedal, but you might not need as much boost to get the job done. JJ/Tesla ECC83S Clean: Wow. A fairly dramatic volume increase from the SS preamp, and the amp sounds bigger and fuller. High harmonics jump out and double and triple stops way down low (below E) are separated and focused. JJ/Tesla ECC83S TubeDrive: Wow redux. In the music I play I dont have a need for distortion or overdrive very often, but this tube makes the TubeDrive circuit work so well, Ill be tapping on the left button on the footswitch a little more often now. Sounds like the Clean tone, but with higher gain and more edge. Very responsive to dynamics. So for me, with my gear, and my ears, the winners are: 1st Place - JJ/Tesla ECC83S. An all around performer and a great value. Its a fine addition to what is, IMHO, an already great amp. 2nd Place - Electro Harmonix 12AX7. 90% of the JJs mojo. 3rd Place Tie - Sovtek 12AX7, GrooveTubes ECC83. Generally OK, but disappointing overdrive performance, and the GrooveTubes was more than 50% more expensive than any of the other tubes. Your local pricing may vary. Sorry for the long post, I just wanted to be as complete as possible.