having some fun with mic theory and algebra....
the mid card is (R+L)
the side figure of 8 is (R-L)
m-s decoding takes the side and feeds the straight signal to the right and the inverse signal to the left (the absolute polarity depends which way your figure of 8 is pointing).
Your two channels are:
card + fo8
card - fo8
(R+L) + (R-L) = 2R
(R+L) - (R-L) = 2L
And that's where the left and right come from.
If you were to swap the mid and side (i.e. plug the card into the side and the fo8 into the mid) then the m-s decoding process would do this:
fo8 + card
fo8 - card
(R-L) + (R+L) = 2R
(R-L) - (R+L) =
-2L2 catches:
First - the notion of +/- is really about phase. A +1 and a -1 are the same 'loudness'.
Second - the above simple algebra assumes equal strength signal when in fact you use much less side. If side was at 50%, you'd end up with:
1.5R / 0.5L = absolute signal level of 2.0
0.5R / -1.5L = absolute signal level of 1.0
That would explain why turning up the side channel causes the level difference between left and right. It seems that the more you dial up the side, the more the left will drop.
Nick - it might be worth swapping the mid / side into the preamp and seeing if you get even gain in both channels as you turn up the side. That would prove it.
david (living in the world of theory)