Originally Posted by PaoloJM

Cons of the 800 2203 compared to the 900 4100

1. It's a single channel amp
2. No reverb or effects loop (the 2205/2210 has dual channel switching, reverb and effects loop)
3. No power switching, could be fitted if really wanted
4. No line out, but one could be fitted if you really wanted
5. No valve failure LEDs, kinda nice to have but I can usually tell when mine have failed as the amp doesn't work!!
6. Less gain on tap

Pro's of the 800 series

1. Less crap in the signal patch, transitors, op-amps and such-like
2. Eat's boost pedals for break-fast
3. Old school power section with a proper choke for B+
4. Easily modded for more gain
5. It just f**kin' sounds better!!

This is just a short-list of the major differences as I see them, there are many, many other minor differences.
Hope the last point didn't get too technical.

The 900 seems to have been designed to encapsulate all the mods that people were doing with the 800 series in one all signing, all dancing amp. They didn't do a bad job at all and it is a fine amp. I can't help thinking, though, that it's a bit over engineered and lost some of it's focus along the way.
The 2,000 I haven't used nor do I know much about it to comment.