I just got the JamMan ExpressXT and plugging it in (using 9v battery), all three LEDs light up and can't record, dub or play loops.
The JamMan Express XT is notorious for chewing through batteries pretty quickly because of the amperage requirement. Unfortunately it has never worked very well with batteries a lot of the time.
Can you remove the battery and power with a 9VDC Adapter with about 200mA of current? Let me know if it returns to operational state