Don't know if you still need an answer, but....
The AH1200SM is;
600 watts rms 1200 peak at 2 ohms on each channel.
Which makes a TOTAL of 1200 rms and 2400 peak watts at 2 ohms.
Trace cabs rated for 8 ohms allowed you to run up to
4 cabs on each output.
Yeah - eight 1088 cabs or 64 10" speakers
Can you say Bass Moves Me, and everything around me?
Anyway, it's not stereo or biampable. You do not need speakers
to run it for D.I use.
AND YES, you will lose volume as your load impedance increases.
2ohms 600, 4 ohms 425, 8 ohms 300 watts estimated.
There were only 10 or so made according to the tech I spoke with at
Trace Elliot in the UK in 2000.
You can use the AH600SM manual (any GP7 manual for that matter) but keep in mind it's a 4 ohm min load and one channel where the 1200 is 2 ohm and 2 channel.