I fixed the bugs in that patch xy_to_amp.pd.....can't believe I was trying such a dumb approach (perhaps I should believe it). This one works! You may want to modify it, a little, because I have set it to trigger calculations whenever x changes or y changes. This could result in a redundant calculation, if both x and y are provided (y first, then x). You can just remove the [f ] on x (left inlet), and the [t b f] on y (right inlet) to change this to trigger a calculation on x, only.
Chuck