The idea would be to shoot stereo Path A not to the 688, but to the 651, as the 4000.
You're then summing the whole console directly in to the 026 cards Transistor pair., which is the sound of the 4k :-))) The 688 summing doesn't have the same mojo.
Looking at the manuals, it's all pin compatible. You'll need to redirect cable S5E, so rather than feed to the 688, it goes directly to 651 S5E input.
Mix A feeds to: LF and RF, and if two more 026 were fitted, Mix B would end up LB and RB.
This would be quick to try, you may have to power cycle the 651, to avoid the 026 cards going into oscillation when changing the cables.
