Having looked closely at Doomie's data, there's one thing I'd like to confirm first: what devices are those two channels set to play on?
I had all devices enabled on channels 0 and 1. (initial voices chan0: 1 chan1: 3)
0x0000 00 01 FF 03 FF 00 00...
Disabling Adlib on chan1, with no other changes alters the 5th byte
0x0000 00 01 FF 03 FB 00 00...
So the Initial voices word comes before the device types I guess and there is 00 at the start.
Edit: Just an observation, but that was with a SCI0 patch altered in Soundbox. I can replicate the same results for SCI0 patches right in SC by disabling Adlib for chan1.
In SCI1, If I try disabling Adlib in a random PQ3 sound patch within SCICompanion, the header is drastically different. In the attached pic the left is with Adlib disabled on chan2 and enabled on the right.