I have been playing on a Fender Mustang II amp for a while now, and I haven't upgraded my amp since I bought my starter amp. I am trying to get that Marshall Tube Amp sound, but I don't know too much about different amps, etc. My price range is ~$400 to ~$600 (maybe a little higher if it's really worth it). So far, from my research I like the sounds of Marshalls, and I probably want a tube amp.

I am not looking to play high-gain metal, mostly light to moderate overdrive (classic rock kind of stuff). I also want a nice clean channel. I have an Ibanez Tube Screamer TS9 if that helps at all. The sound I am looking for is tight and bright, which isn't happening with my amp right now. I do not plan to play at huge clubs, mostly my house, with my band, and at small events every so often.

Also, I have asked a lot of advice from a lot of people, and they talk about cabs, heads, and combos, and I don't really understand what they mean, so if you do recommend something, try to explain the language you are using.
Have you used the Fender Fuse software with your amp yet? I can't imagine you not getting the sound you want if you tinker with the software and modify the presets. You can get some very nice cleans with the Fender Twin Reverb model and the Vox and Marshall models can be modified to create some nice gain sounds.
Well if you want the Marshall sound, then get a Marshall.

Guitar Center has a few used JCM 2000 DSL 401 combos for under $500. I believe you want to check the year for the output transformer overheating but just google it.

Head = Pre-amp and Power Amp in the same box.
Cabinet = One or more speakers in the same box.
Combo = Pre-amp, Power Amp, and Speakers in the same box.
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .
Quote by diabolical
Marshall DSL40C, Traynor YCV50B, Carvin V3 should work.

+1 for the Traynor