I use a solid state pre with my GT se-40, mostly because I wanted a remote. You may want to consider having some mods done to your amp, such as those offered by sonic craft, see here:
http://www.soniccraft.com/upgrades/se40.htm
I had the SE40BB upgrade done at about $200 including labor by just shipping the circuit board to them. This upgrade improves the sound while also removing some of the problems some people have had with it, as explained on the site. Well worth the effort and the turnaround was pretty quick. They may also have suggestions on a tubed pre.
The best 6L6 I found is the Svetlana, rather than the Sovteks supplied with the amp. The best SN7 is a tossup between the EH and Sylvania.
Best of luck with your system.
http://www.soniccraft.com/upgrades/se40.htm
I had the SE40BB upgrade done at about $200 including labor by just shipping the circuit board to them. This upgrade improves the sound while also removing some of the problems some people have had with it, as explained on the site. Well worth the effort and the turnaround was pretty quick. They may also have suggestions on a tubed pre.
The best 6L6 I found is the Svetlana, rather than the Sovteks supplied with the amp. The best SN7 is a tossup between the EH and Sylvania.
Best of luck with your system.