Matched interconnect cables producing different volume left and right

This one's got me stumped.
I recent rearranged my listening room and needed much longer interconnects between by preamp and monoblocks.
I had an old pair of 4 meter cables from about 1994 stored away (very expensive back then) and pulled them out.  They are pure silver with a copper braid with WBT RCAs on the sending end and Neutric XLRs on the receiving end.  I needed XLRs on both ends so I soldered on a set of Cardas XLRs on the sending end.  I understand intellectually how to solder correctly but have very few reps as I seldom need to do it.
When I put them in the rearranged system, the sound stage was much worse than before and the left channel sounded a little louder than the right.  I switched the cables left and right and suddenly the sound stage was equally disappointing but now switched so the right channel was a little louder.  No obvious distortion or anything like that from either channel.
So here's the question, is it possible my amateur soldering significantly changed the resistance (or some other property) of the cables enough to influence the volume?  Or is it more likely that all this aging changed some important property of one cable?
How do I figure this out?  These things cost way too much to throw them away.