Hello, I popped the grill off my SL5 today because I recently bought a couple JJ ECC83 preamp tubes. A week ago I replaced the preamp tube on my Blackstar HT 5 and it really made quite a difference. I figured today was the day to swap out the one in my Marshall. Well that's when I found out the SL5 has 3 preamp tubes and thanks to information on this site and others I know which one is the V1 (the only one under the metal cap) and the one I want to swap. Reading about the cover led me to the debate of whether it affects the tone, and whether it should be left on or not, as it's there to restrict RAF interference. I also learned that the yellow printed Marshall ECC83 90095 I took out could possibly be a JJ too. Oh yeah, and the question of how you get the cover off. After about 20 minutes of actual work and two hours of over researching, I'm done. I decided to leave the cover off. Now if I think it sounds better, I can't really say whether it's the tube swap or removing the cover, or a combination of both. To be honest I don't really care, unless the amp sounds like crap all the sudden. I'll report back if anything is significantly different, good or bad. The only question at this point is whether I should try the yellow branded ECC83 in one of the other preamp tube spots, or just keep it as a backup. Just wanted to share this information and pics in case someone else does too much research like I do. here's a bunch of pics of the three preamp tubes I took out; detailed pics of the cap showing what holds in place and why you have to screw it a bit before it will come off; and, that you only have to take the back grill off to get to the tubes. Great amp overall, I wasn't expecting the clean channel to sound so crisp, but the dirt channel is what this amp will be on 99% of the time.