The 900's are basically the High Gain Dual Reverb heads 4500 and 4100 (these amps are what the 2205 and 2210 evolved into) and the 2500 and 2100 models (that are what the 2204 and 2203 evolved into). The SLX was a further revision of the latter. The Mk III designation followed suit because the 2203 and 2204 were Mk II amps.
The 900's had more gain on tap than the 800's. This often turned into fizziness in the wrong hands. Even in the right hands the fizziness is something you have to deal with. This is where most Marshall fans draw the dividing line between the amps.
Heaviness in a Marshall is all in what you are playing and how you are playing it. Sure you can get a great tone to start with but your attack will be what varies the heaviness. If you are looking for a deep voicing like a Rec then you should just get another Rec. Marshalls just aren't voiced deep like that. I would pick a JCM800 to match up to a Rec for contrast. Marshall's modern high gain kind of sucks because of the fizziness. I would consider a JCM800 as a vintage high gain and a more appropriate matching. The tone is better in an 800 than in a 900. As far as what 800 to use goes, it would depend upon what you are playing. The single channels are classic and very useful for a set tone. The split channels are slightly more gain and have the option to use reverb and a less distorted channel via footswitch. Obviously, the earlier 6 cap vertical input is the 2203 you want if you go 2203. You can even get a JMP version. The 2204 didn't receive any real changes even in the later horizontal inputs. Again you can get a JMP version too. The split channels were revised fixing channel bleed and not so good distortion of the earlier models. This happened around the year 1986.
I hope that helped. You would have to play the amps to really get a feel for them.
If it makes any difference, I sold my 2210 to get my 2 ch Dual. I still have my JMP. I don't expect the JMP to get real heavy but I know what to do and how to play if I need it to sound any heavier than it is. My rig before was the 2210 and the JMP. Obviously, the 2210 was covering the higher gain stuff. The 2 ch Dual handles those duties well enough that I don't miss the 2210 anymore. What the 2 ch Dual doesn't cover the Mark IV does and I still get my Marshall fix from the JMP.
BTW, a 2203ZW is just a 2203 RI with cosmetic changes.