All are mono recordings, I never heard of any stereo recording of Furtwaengler.
About the recording effect, EMI's mono of 50s is very famous, he also was
proud of its mono recording, so he published his major stereo recording until
1958 (Oistrakh's Beethoven violin concerto). But I don't think recording
effect is everything, someone may say that good recording can hear the
details, but I think most important thing is the inside, otherwise E.
Caruso's singing art can't be remember so long.
--
Truth, Science, and Music are all I
believe and love.