Please excuse this one off topic question, but am I correct that if I am still running High Sierra on an older mac that I am out of luck for updating to the newest version of HQP? Thanks.
Yup. I’m in the same boat. I just recently asked jussi about this.
I’ve actually found on 10.13.6, v3.25 hqp with a i7 4790k and a TitanX I am able to run the extremely demanding poly-sync-xtr ASDM7 @ 44.1k 256 if I check multicore dsp and uncheck CUDA. Processor usage is at ~40-44% usually. Even if I could upgrade to v4 on this machine I can almost guarantee it will not work with the new EC modulators.
My findings for hqp v4 on win 10 core i9 9900k with a titan x. The CUDA in titan x is not powerful enough. But I did get all filters working for me at DSD256 playback.