Small update. There is no CUDA switch on the 700 board, but I reset the logic board again. Taking a cue from the solitary appearance of Sad Mac (software and bus errors), I removed the HDD to an external enclosure, checked it after attachment to a 9600/400, reinstalled Silverlining 6.2.1, ran DW 2.1.1 and PlusOptimizer, and booted the 9600 and a Quadra 950 (without the 601 card) from the drive. The 9600 ignored the 601 control panel, as expected. I checked the Diamondtron against other Macs, and checked the absence of raster with an attached Color Plus 14.

The OS 8.1 installation (for any Mac) had placed Power Macintosh Card 1.0 and 601 Processor Upgrade 1.0 on the HDD. I have used only the former, not least because the latter 'cannot be used on this computer'. I also have the DayStar 3.2 software installed, but not active.

At this moment I lean to the possibility that logic-board video is the culprit, which I shall have to test.

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(60x) 13DT + 3PB + PTPro; (G3) 7DT/MT; (G4) 3T + PB. System 8.1 to OS 10.5.8