I'm not wholly against graphs but I still, as a devil's advocate, ask why you need them? And to say "to confirm what I hear" is moot for you have already confirmed it by the fact you heard it.
The only thing the FFT - as a limited testing procedure - can show you is that there are harmonics or not. It won't show you how you hear the quality of something. You won't tell you anything about quality.
Regarding the FFT flaws: put 2 sine waves through said processor one at 110 Hz and another at 910 Hz and you will get a different response. What was gained? (Check the nauseating "Let's Do it: The Ultimate Plugin Analysis" thread for my expiation on this matter.
Regarding quality: Take for example a UAD EQ like the 1073. Try if you will to null it with a standard digital EQ and it will not null 100%. Yet, there are no harmonic distortions going on. HD is not the end all be all when it comes to "character" or asserting "flatness". Looking at an FFT plot won't make that distinction. Again, listening wins.
Again, to simply check for harmonics is fine, or if you will, "geeky" investigation/curiosity but not for checking "quality" or getting the ultimate truth about a given processor. Quality comes from qualia: the experience of the given sense for a given endeavor e.g. hearing + audio. I'm just saying, this thread is en route to nit-picking but do what you will. You aren't going to gain anything further than what your ears have told you already. The paradox being, if your ears didn't tell you then there is a huge question mark against your vocation of choice here!