Peter :
The 2400 has a power amplifier built in. I would suggest looking at the Beocentral article here. Tim knows a lot about these decks - the 2400 was mine for a while!
As far as I can see from the article, the only difference is power amp and 'mixing facilities'.
Is the power amp really necessary ? If connected to a Beolab 5000...
What is the mixing facility ?