Good morning. You were all right! Thank you so much, I appreciate your help.
My Modwright is an older model without the high/low pass switch and has the L and R terminals oriented in the same direction rather than mirrored like the more recent ones. I put my cables on by feel and, without a manual available, was using one the on-line version which had the mirrored arrangement. So, the polarity was reversed on the left side.
Anyway, blessings and thank you's - enjoy the tunes and the long weekend!