@Tamasg The SVF makes a lot of sense, they modulate well and direct form biquads don't. If I stay with formant+bandwidth I'll also switch to that. However, I'm currently exploring line spectral pairs so might not.

I'm also wondering if you should drive this at a higher sampling rate. The BLT gives you frequency compression near Nyquist, which is not present in the analog source. I think you'd want to add an additional lowpass around 4-5kHz for voiced.