While you're waiting for the new tubes, check out the rest of your rig first. If you have any alternate preamp that you can use, that would be the first thing to try. If you've got another amp with a preamp out jack, try hooking that up, or try this out with your PA board (you can use an effects out or line out if you have gain controls on your amp). Use your normal cables if you can. If things sound weak now, then the problem is more likely your power amp.
If that's working properly (reasonable clean output level), then you can dig in deeper with your preamp. Preamps tubes should last a long time (unlike power tubes) but are subject to damage from impact or vibration. The most common failure mode is that the output gets wimpy over time, so this may well be your problem, however, if you put another tube in and it's still bad, then perhaps not. The 12AX7 is the most common preamp tube out there, so you should be able to mooch one from your guitarist to test.
There's a spring-loaded metal shield that covers the tube. Is this the sleeve you're talking about? It serves 3 purposes - first, it is an electrical shield that helps keep the preamp quiet, especially from the EM field from the power supply in your preamp; second, it helps keep the tube seated in it's socket; third, it provides some physical protection, not that there's much that going to hit it inside the case. Tubes vary in diameter, so if the new one is too big, you can go without it, but it would be better for you to find a tube that does fit in the shield.
Good luck,
David Fung
