For the G68 series, I believe you can just plug USB directly into the USB Maintenance port.

Possibly this was only enabled in the later models?

1. Trinnov Alt32, Meadowlark Semi-custom 4-way Digital mains, Custom 3-Way Digital center, M6, DSP420, HSU VTF2
2. G68D, DSP6000, DSP5kC, DSP420