I know I sound like a broken record here, but why don't you just use WHDLoad? Although if you really want a "classic game experience", go back to 1.2 or 1.3, lol.
Also, I don't think the Amiga mixes audio channels. Plug in one channel, you'll get one channel. Although maybe something newer (like a 4000) would do it.
A1200 does mix audio channels if either L or R is connected. Presumably A600, A4000 and CD32 will behave the same...but that's just a guess on my part.