From Nelson Pass I received an excellent tip that applies to this or other setup:
Connecting preamplifier balanced output to the amplifiers unbalanced inputs: pin 3 for Amp1, pin 2 for Amp2; both share pin 1 ground; phase of amp1 inverted at the speaker output.
If Amp1 has more gain than amp2, put pin 3 into a 10K pot.
Voila! Simple and brilliant.