Vintage amp problems

(not sure this qualifies as high end audio...)
recently purchased an old sansui Au-555a amp to run my same age Pioneer CS 88a's
left channel works fine, right channel not so fine
the right channel sounds like the speaker has lost all definition
have tried the following:
1 - swapping speakers
2 - changing speaker wire and length (tried with the shortest length possible...)
3 - changin imput and input leads
4 - trying speaker set b as well as speaker set a
5 - standing on my head while listening (that really didnt work)

net result is regardless of all of the above, right channel sounds very undefines and distorted

any clues as to where i should start?