is it like the clean channel on the 2 channel JCM 800? does it distort when turned up loud? is it a good clean?

Depends which one you are talking about. There are many different types of JCM 900's as there are 800's
Jackson DK2M
ESP LTD M-200FM w/Tone Zone + PAF Pro
Ibanez RG7321
Digitech Whammy IV
Digitech GSP1101
Furman M-10 LxE
Peavey 6505
ISP Decimator