Hi @peppermint, I think the issue is the use of dBu on the tests.
I have made a fix and released a special version where just the “Amp THD versus Input/Output Level” plug-in is active. This has fixes
https://cdn.shopify.com/s/files/1/1631/5609/files/setup_QA40x_1.217.exe?v=1749825335
If you set your output gain to 6.02 dB, then your max output should be settable to 18 + 6.02 = 24.02 dBV. And then, if you ask for 25 dBV, you’ll see:
Similarly, in dBu mode, the max output will be 18 + 6.02 + 2.22 = 26.24. So, a request for 26.0 dBu will succeed:
When time permits, will you see if this special version works as you expect? If it does, then the changes will be rolled into all the plugins.