Thank you for you response.

I'll have to play with the musicmatch encoding settings.
Currently they are 160 bps, all other settings default,
but clicking Options Settings Advanced gives some
MP3 encoding settings such as Processing Level which
is currently at Normal. I think Very High might preserve
the stereo separation 100%, will have to see.
There may be other parameters to tweak as well,
that resulted in some loss of stereo separation.

Anyone have any ideas?

I have all my audio as WAV files also,
but the size makes it prohibitive to making multi backup copies.
I'll have to explore the settings and other codecs (such as LAME)
and go a little higher on the bps (prolly 192).

Thanks to everyone else also for your responses.