Bell MXA-1 Multiplex Adaptor outputs tinny sounding audio when hooked up

[vortalexfan]

Hello Everyone, last week I finished up fixing up a Bell Carrilon 6060 Amplifier and 6070 Tuner with the optional Bell MXA-1 Multiplex Adaptor installed in the tuner and I got the tuner realigned properly (as it was completely messed up because the AFC Circuit when engaged wasn't locking into the signal properly like it shouldo of).

But for some reason I couldn't get the multiplex adaptor unit to sound very good when it was hooked up to the tuner and the amplifier, the audio coming out of it sounded almost like you were listening to someone singing in a sewer pipe or something like that, it was very tinny sounding.
It lacked bass, no matter how much you turned up the bass and the treble sounded super shrill and didn't sound very well defined (it sounded almost like you were listening to an old Lo-Fi radio from the 1950s, except it was stereo instead of mono.

Any ideas as to why a factory add-on Multiplex Adaptor would output such bad sounding audio?

Thanks for your help with this issue.