what to buy under $500 bi wire spkr cable

What speaker are you using. Perhaps the problem isn't the speaker, not the cable.

It is not the cable although the terminations may be corroded.

Just to be certain that nothing simple is being overlooked, are you sure that the + and - connections to one speaker are not reversed, relative to how the other speaker is connected? A reversal of relative phase would probably account for both of the symptoms you are describing.

Regards,
-- Al