the other thing i should say about setting high rates is that i just wouldn’t do it if you don’t have to, particularly while writing, because it’s just expensive. if rate is, say, 63.9, then each sample of processing will require 63 or 64 interpolated writes to the buffer, and these are the costliest operation.
since you only have 2 voices you should be ok pushing the envelope of performance, but in general i’d limit rate modulation to 3 or 4 octaves to avoid dropouts. (not crashes.)
the crashes are almost certainly from exceeding the hard limit on rate, which will smash memory since we have traded some safeguards for speed in the inner read/write loops.
