This has two monoblock amps. Its a digital amp so you can run one amp and not the other without harm. Theoretically each amp runs 250W at 8 ohms and 500 watts at 4 ohms. Two outputs one for each amp. You could connect one 8ohm cab, one 4ohm cab, two 8 or 4ohm cabs, or one 8 ohm and one 4ohm cab. Minimum load is 4 ohms for each output.
From the manual: "1000 WATT - The two SPEAKER OUTPUTS on
the 1000 Watt model are driven independently by 2 x 500 watt output stages. Each socket can run into a 4 Ohm load for maximum output power."
If you look for Talkbass threads on the amp its failure in output is connected to the preamp section, but I don't know the details.
I own this amp, and used it regularly (till I got my Eden WT550 which is currently my go to amp for electric gigs) with two Schroeder 8 ohm cabs (a 1212L and a 1215L) for outdoor gigs with plenty of oomph and indoor gigs with just the 1215L just fine. It may not have lived up to its full 1000watts running two 4 ohm cabs, but I never felt like it did not deliver, and the band I was using it with was very loud. And I really like the tone coloration of this amp on the low end....