Right now I run my Marshall JCM 2000 DSL 401 Combo through a Marshall cab that isn't mine but is at our practice space. The cab provides added volume but I primarily use it to get a beefier low-end sound that my 1X12 combo can't give me. I typically play on the clean channel and use the MXR Micro-Amp and USA Big Muff. The tube sound is classic Marshall, and when the tubes are driven to 10, it screams pretty nicely. I can't complain too much about the quality when the amp isn't acting up. Right now, however, its just not the precise sound I'm looking for.

Additionally, the Marshall combo has been a diva lately with the typical overheating issues, and I'm not entirely happy with my current sound. So I'm debating whether to invest the time and money into furnishing this amp into my rig of the future or selling it and try to turn a profit (got it for cheap) and starting over with a new amp. Essentially my choices are:

1) Refurnish my Marshall DSL 401 with a new speaker (Celestion V30 or Hellatone60 have been recommended), spend some money on some cool pedals (possibly an EQ, MXR blue bog, Digitech Whammy, a better Fuzz than the US Big Muff, octaver, etc. etc.), and buy my own cab.

2) Sell my Marshall and buy a new Combo in the $500-800 price range. I'm looking to buy used, and from what I've seen/heard, a Fender Twin Reverb or Blues Deville may work, but Ampeg, Orange, Vox may be too expensive.

Basically, I'm looking for some advice on whether to stick with my Marshall which I got for a pretty good steal and putting money toward pedals/cab/upgrades, or use that good deal to sell the amp and turn it around into something else. Also, I need some advice on what brand of amp and what type of pedals I should be keeping an eye out for. I'm fairly confident/proud with my skills at guitar and have been playing for 6-7 years, but I'm not very knowledgeable at all in terms of how gear works, the quality of brand names, or what gear is best suited for me. I'm a graduate student in dental school, so all my free time not working in people's mouths is devoted to practicing, writing, or performing, not becoming a gear head.

Any suggestions would be awesome.