I imagine most people who've gone to the trouble of wiring up an impedance measurement jig to complement their FR etc. measurements (if you haven't, see here) have already made some sort of variable-OI calculator to use with it, but cursory googling didn't turn up anything publicly available. Tired of doing the calculations by hand for only a few points on the curve I wrote one myself, and I'm pleased to share it with you lovely people: https://www.dropbox.com/s/cxjar2y10pqvfdn/Frequency response vs amplifier output impedance calculator.xlsx?dl=0 Let me know if the link doesn't work for you, I'm not used to this public sharing business. I/O and usage notes are on the first page. The results assume the input measurements were taken with an amp of zero Ohms OI, which is of course not the case but if you're using a sensible modern solid-state unit (and some tube ones) the output will be below 1 Ohm and the results will be close enough. If you're obsessive, you could adapt the spreadsheet for the particular amp you're using. Or otherwise modify/improve it, adapt it for other software (ARTA or whatever), as you wish. To show you that it works with at least one example, here are results for the Westone UMPro20 IEM: The red frequency response curve shows underwhelming mids & lower treble, but the brown impedance curve suggests an amplifier with high output impedance might partially cure that. Calculated results for 3 cases below: The mids are certainly helped, but even the 120 Ohm result indicates lower through mid treble still won't be great (on my home-made tube coupler more neutral things don't start dropping off until after 5 or 6k). Anyway, I hope somebody will find it useful. Please let me know if you find mistakes. Regarding which, the usual disclaimers apply: I will not be liable for any injury or death, etc. etc.