I had this same problem last month, and after a bunch of searching and calls to AMX here you go:

4800 baud, 8 bits, no parity, no handshake
All characters sent in uppercase
Command sequence does not require carriage return

Power:
zone 1 on = Z11
zone 1 off = Z10
zone 2 on = Z21
Zone 2 off = Z20
etc
all on = ZA1
all off = ZA0

Sources:
z1 source 1 = S11
z2 source 4 = S24
easy!

Volume:
volume range is 00-31
zone 1 volume 15 = V115
zone 3 volume 8 = V308

volume ramp up zone 1 = V1++
volume ramp down zone 3 = V3--

Function Keys:
Fast forward zone 1 = F1F

F = >>
R = <<
S = pause/stop

there you go.

e