[Leffmann]

Try this:

AM_NORMAL = 0
AM_NONE   = 8
AM_23     = 4    ; channel 2 modulates channel 3 volume
AM_12     = 2    ;         1                   2
AM_01     = 1    ;         0                   1

  move.w  #$f, $dff09e
  move.w  #$8000|AM_01, $dff09e

  move.l  4, a6
  move.l  (276, a6), a2
  lea     (92, a2), a0
  jsr     (-384, a6)
  lea     (92, a2), a0
  jsr     (-372, a6)
  move.l  d0, d2
  jsr     (-132, a6)
  move.l  d2, a1
  jsr     (-378, a6)
  rts