Karl Miller asks about well-known people who were chamber musicians.
The name Raoul Berger- not exactly a household name, I realize- comes
to mind.  He was a pre-eminent legal scholar who wrote a famous book
entitled "Government by Judiciary" which in meticulous detail described
how the Supreme Court of the United States subverted the expressed intent
of the framers and adopters of the 14 & 15 ammendments to the US
Constitution, and his arguments have never, to the best of my knowledge,
been rebutted.  [Americans simply accept that the Supreme Court has
subverted the constitution they are sworn to protect] Berger, who
immigrated to this country from Russia or Ukraine as a child, was a
professional violininst early in his career, but changed careers in his
thirties, partly, I gather, because of the economic challenge of being
a musician during the Great Depression.  Unfortunately, I cannot say
how- or even if- he was active as a musician after he gave up his
professional career, but that is something you could look into.

I also might respond, belatedly, to a comment made on this List about
Albert Einstein.  The comment was that he played the violin rather poorly.
My impression is that he was a rather good amateur, and that professional
musicians who played with him said he could have joined their guild had
he been so inclined.  I cannot vouch for the accuracy of my impression
here, as I may simply be buying into an Einstein myth (if so, I really
don't care to be disabused of the belief, however).

Finally, I have to mention Richard Feynman, with hopes I can scoop Dr.
Chasen on this.  Amateur painter, (very) amateur author, and amateur
drummer that he was, these side accomplishments all added to the enormous
aura that surrounded him.  I guess qualifying a drummer as a chamber
musician is a bit of a stretch, as I've never heard of a drum quintet.

Hope I've succeeded in muddying the waters for your research!

Larry