vintage little big muff bypass issues

[zohany199769]

hello. 4 yrs ago i purchased a vintage little big muff from about 1978. it works with the usual issues (switch needs replaced) but does something strange that also renders it unusable with any other pedals. when the fuzz isnt engaged it boosts the clean signal of the guitar just enough that i have to chuck out all my boost effects(which are the backbone of my tone and feel) . so my question is, does anybody know what could be causeing this to happen and any mods could do to fix it. its layout is basically a late 70s op amp muff. just stripped down . any help is much needed.