Help with Vintage B&K Power/Pre Amps


I have a ST2140 power amp and a PRO 10MC Sonata Series preamp... thought I had trouble with one of my tweeters (right speaker sounding overly bright to the point of listening fatigue)... a friend brought over a pair of B&W monitors and after plugging them in, the right speaker had overly bright highs as well... anyone have any idea what might be causing this... do I need to send the 2140 into B&K to diagnose, or maybe the PRO 10MC, or both... any suggestions appreciated... this journey down the vintage equipment road can be challenging... thanks in advance.
tapar1
Tapar1, Is your the Pro10 preamp the model with the red/blue or black/gold writing on the black faceplate or the machined black faceplate with grayish inlaid color where its sort of recessed? There were early and late generation pro10s.

Depending on which version pro10 it is, if you decide to let go of it, I might be willing to take it off of our hands if the price is right and to fund your next preamp purchase.
It's the one with the red & blue on the faceplate... just sent it in for servicing... not sure if it is the early or late model...
Hopefully this is not to dated of a response.  I have the same Pro 10 preamp that I took out of a closet after having been stored of about 15 years, cleaned the contacts and connected it last night.  At first the right channel was lacking in high en response.  I tried different input methods, changing cables and continued to have the same result.  I then connected my output to the second set of preamp output and the proble went away.  I guess its limited to the right channel on the preamp output A.  Doesn't solve the problem but at least I am enjoying the preamp again.