I’ve owned several preamps that had bleed through, (just part of the design) as Al stated. Shorting plugs did indeed solve the problem.
http://www.tweakshop.com/Shorting%20Plugs.html
http://www.ebay.com/itm/like/121448543140?ul_noapp=true&chn=ps&lpid=82
http://www.ebay.com/itm/like/121448540481?ul_noapp=true&chn=ps&lpid=82
NOTE: NEVER use these on OUTPUTS!!! Inputs only.
http://www.tweakshop.com/Shorting%20Plugs.html
http://www.ebay.com/itm/like/121448543140?ul_noapp=true&chn=ps&lpid=82
http://www.ebay.com/itm/like/121448540481?ul_noapp=true&chn=ps&lpid=82
NOTE: NEVER use these on OUTPUTS!!! Inputs only.