Something seems not right there - you shouldn't need to power a cabinet to be able to hear sound from the XLR outputs.
What radatats said is correct - be very careful about what you plug in where! The XLR 'studio' outputs, as near as I can tell are "line level" sends that could be connected to a mixer, etc.
The "live" 1/4" jacks are *definitely* sending power, they are intended to be plugged into a non-powered cabinet / stack, etc. - but definitely NOT a combo amp that already has a power amp of it's own.
You should be able to get something out of the 1/4" FX LOOP send, at least to sort out if the unit is generating any sound at all.
Does it look anything like these photos, on the back of the head?
From the manual that radatats linked to:
"TAKE A LOAD OFF
Operating the Flextone II HD without a load (that means without speakers
attached) is OK! You do not need to have speakers connected to the
amplifier in order to use the balanced Direct Outputs (you wouldn’t want
to run a tube amplifier without a load)."
– Slide the Live/Studio Switch to the Studio position, and the XLR
Outputs will be +4 line level outputs with A.I.R. speaker-mic-room simulation,
ready to plug right into your recorder to lay down your guitar parts. No hasseling
with microphones (or neighbors annoyed by the ruckus) required! The speakers are
disabled in this mode.
– In Live Mode, the XLR Outputs are mic level, ready to plug in to the
mic snakes you’ll probably find at your gigs. The direct out signal is speciallytailored
for live sound and the speakers, of course, stay active to rock the house."