The only bad thing I can say is the rectifier needs upgraded along with the capacitor. I did mine a while back due to a volume fluctuation.
I'm not going to go into that due to it is another topic all together.
Regardless The Marshall JCM 900 SL-X 2100 is and always has been a BEAST of an amp when it comes to a high gain guitar amplifier. I ENJOY MINE DAILY.
Great amp. Mine is a 1993 2100 SL-X. EL34. Run it through my '89 Marshall 1960TV, '97 412 bottom (G12Ms) or a custom Carvin 212E w/G12H75s. I run it straight with my Les Pauls. Sometimes with a wah. Here is a taste of the SLX.
This is a 100W tubeamp (5881 tubes) from the nineties. This is a pumped up version of the much more common JCM 900 model, with extra tube on the preamp section. Respectively it has less features than basic JCM 900, just one channel and no reverb. The controls are kind of simple on the surface: bass, middle, treble, presence, two volumes (switchable with foot-pedal) and exotic system of two different gain controls labeled “Preamp Volume” and “Gain Sensitivity”. After using this amp for over 15 years, I am still not sure what the hell the labels mean, I just know how to tweak them by nature by now, other one basically controls the basic preamp gain and other one gives more gain on top of that. The other knob actually goes up to 20 (beat that, Spinal Tap). As typical with tube-Marshalls, tuning one knob seems to affect other knobs settings too, so you have to experiment a bit to find your sound.
This amp delivers classic sweet “Marshall sound” if you just know how to tune the gain settings. Kiss used these amps on their first reunion tour with Ace Frehley and Peter Criss, the basic sound fits that kind of not-too-distorted hard rock sound really good. If you are interested playing cleaner sounds too along with the Marshall sound, you need to tweak volume on your guitar to get lower pre-amp gain from the amp. I do this all the time and it feels like a second nature for me, I almost don´t even miss a clean channel. If you play metal, this amp probably does not give you enough distortion without using pedals in front or totally blasting away with volumes on ten to get the tube distortion (I have experimented with that too :)). But with decent overdrive pedals, you can get pretty much enough distortion you need. This is a classic hard rock machine.
Modern Marshalls (eighties and later) don´t have a good reputation on being durable, and this amp has seen the insides of amp repair-shop at least couple of times too. There are just some electronic components which fry out after enough years, but luckily they are easy to replace and are not too expensive to fix. This amp has survived many gigs, many falls, many spilled beers on it and the basic construction is very dependable. This amp has once fallen from top of full stack of speaker cabinets (with power on and tubes heated) and it performed a gig later that night with no problems, so the only build quality flaws are some cheap components on the circuit board. And of course you have to change the tubes from time to time, as with any pure tube amplifier.