Yes, they're pretty good. Classic Records managed a pretty balanced frequency response even in the 33 rpm issues.
"Can anyone offer input on the RCA Heifitz recordings on Classic records?"
The best of the bunch sonically, for me, is the Hendl Violin Concerto on LSC 2734 - really superb!
These are also quite good sonically, if not quite up to the standard of the Hendl recording:
LSC 2435 - Sibelius Violin Concerto
LSC 2652 - Bruch Violin Concerto
LSC 1903 - Brahms Violin Concerto
.