I measured the frequency response with the different settings, so you can predict, how they might affect a specific headphone model.
The Harman options increase the ear gain area. For example,
L peaks a bit earlier than
M, while
H mainly boosts the presence region. Activating all of the settings or using them as pairs seems to work a bit differently.
Super Flat is just an upper treble boost, which starts to kick in at 8 kHz. Can be useful with Crossfeed settings, which all increase the bass.
Crossfeed of course does other magic as well, but this is the FR change at bass frequencies. Option 1 seems to be the safest and it works almost with every headphone in my collection. For some headphones, the 5 dB bass boost with settings 2-4 starts to be a bit much, but you can always balance that with other settings (Harman & Super Flat)
Fiio FT5 is one of the headphones that really benefits from both Crossfeed 1 and Harman settings. This way it becomes well balanced. Same should happen with Meze Audio Empyrean and T+A Solitaire P, for example, as those are missing some of the ear gain compared to Harman target, but are otherwise quite linear.
I am borrowing the HP-10 Deluxe for a review, but I might buy the base model afterwards (I already own too many DACs). This thing sounds super nice! Compared to Ferrum Audio OOR as an amp only, HP-10 sounds more open and transparent. It also separates treble detail more easily, but isn't sharp or clinical at all. OOR sounds slightly fuller and smoother, but you lose some of the separation. The amp makes you focus on the vocalist and lower mids, while treble stays quite relaxed in the background. I like this, but HP-10 feels more high-end with its neutral and more separated presentation. Everything just sounds more lively compared to a slightly softer OOR. And when I activate the higher damping factor, HP-10 becomes even more dynamic. However, this setting alone increases the volume around 5 dB, so you have to lower it before making any comparisons to the lower damping factor.
By the way, HP-10 works well with Hifiman HE-6 and Mod House Audio Tungsten (Single sided), so you shouldn't need another amp for those.