Based on a test CD track (the Sheffield A2TB test disc), I can hear distortion levels starting clearly at 3%. I'm assuming based on this data, I wouldn't be able to hear distortion on the FV. The same data shows with impedance multiplier off, same 32ohm impedance and low gain, the THD+N ratio is about 3%.
To be honest, I can't tell a huge difference with the impedance multiplier on or off. With complex passages, I do hear some fuzziness/congestion with the Diana V2 with the multiplier off. My plan is to start with the lowest distortion settings on the FV to establish a baseline and work from there. I wanted to make sure I interpreted the data correctly.
That's a perfectly valid approach to figuring these things out.
Here's my ritual for when I listen to a new (to me) amp:
1) Plug in the headphones that I like
2) Open my reference tracks playlist in Roon, which is just an eclectic bunch of exceptionally well recorded and mixed music across my usual genres, each track with its own attributes
that I know like the back of my hand and that I can listen for to assess the strengths and weaknesses of a given piece of gear
3) Start playing the tracks and fiddle with the amp's volume knob and all its relevant switches until a setting is found that pleases my picky-A-F ears
And no, I'm not posting this to have the thread degrade into an argument about spread sheets and numbers vs. listening. We've been there. We've done that. And nothing of value ever really came from it. No debate can and should be had over this, as each approach is both utterly flawed and perfectly valid in its own way.
They just lead to different outcomes.
Your approach is the more scientific one, if you will. It allows you to learn something about the interaction between different pieces of gear and transducers, and you will probably end up with a result that is the "correct" one from the perspective of an engineer or physicist, including all that peace of mind that a lot of people draw from knowing that they've done it just the way they're "supposed to" from an engineering point of view.
Meanwhile, and more often than not, I learn nothing of real (i.e. scientific/technical) value from my approach. Well, nothing other than what switches need to go where to make things sound good to my ears. But that only ever benefits me, really, seldom anybody else.
Even worse, my approach has also lead to results that make Jason (as the designer of the amps in question) roll his eyes at me in the "best" of cases, and outright cringe in some of the more … uhm … egregious ones. Like when I mentioned a few months ago that I get this truly magnificent sounding result from using a Piety with my 13 ohm Æon Noirs. Or when I mentioned years ago that I have a pair of (supposedly) 60 ohm planars that sound especially magnificent when plugged into my Valhalla 2.
But then again — and while I know that nothing could be further from the truth — I sometimes think that Jason only added the impedance multiplier to Folkvangr just so that I would finally shut up for once about how much I enjoy plugging my low-impedance planars into everything that comes across my desk, but especially into amps that were never designed to drive them.