Funny enough, I first recommend you don't take it to Guitar Center to have it retubed. They don't always carry the best tubes and only God knows what their "techs" are like-If they even have any. However, you can use them as a resource. I called them and found out who they use for any repair work on Marshalls in my area. Given the agreement they have with Marshall, they have to use a certified Marshall tech and they should willingly give you the phone number for a tech in your area. Also, check the Marshall website and search to find service centers for your amp.
For tubes, SED winged C EL34 for the power amp. For the preamp, it's kinda a personal choice thing. Many people on this forum like different tubes. I use mostly JJ 12ax7's in mine. Although, I recently tried a Groove Tubes Gold Series 12ax7 in the V1 position and like the result. In truth, it's not a huge difference for me from the JJ to the Groove Tube but there are many other manufacturers that could give you a more definative difference in tone, feel, brightness, etc.
Good luck, and read through this forum, there's a boatload of info in these pages on tubes and what they do/how they sound, etc. In the end, you just have to buy some and try them out.
Guitars: Schecter C1 Classic & C1 semi-hollowbody, Ibanez S Series 7 string, Rebuilt, Effed over, semi-destroyed and resurrected Ibanez S Series-AKA "Dragon", Sterling JPM100d Petrucci, Alvarez acoustic, 1971 Geronimo Pena Fernandez classical (Brazil), 1972 Gianini classical (Brazil), 1960's Gretsch Country Gentleman.
Amps: JVM 410H, Bugera 6202 (mod w/ Heyboer OT, and Merc Mag choke), Roland Cube, Mesa 4x12 cab w/ Celestion V-30's.
Effects: TC Electronic Gmaj, ISP Decimator ProRack G, Digitech Whammy, Dunlop Zack Wylde Wah, MXR 10 band EQ.
Recording: Apple Macbook Pro, Presonus Firepods, Apple Logic, Alesis Monitors.
Other: Zyzzx, Mogami and Monster cables, RoadRunner Road Cases, Samuel Smith's Oatmeal Stout, Breckenridge Vanilla Porter.