Ultimately yeah, we need a reference in the same way we need them for weights and measurements. My point was, it is possible to "eyeball it" to a fair degree of precision. Fair being the operative word.
If I tune a guitar with a starting pitch as a reference, a piano note or another guitar's E or A string, I' ll get it sounding just as good as I would using your garden variety bargain bin tuner.. I wouldn't use only my ears for lutherie work such as intonation setting, but for recording? Any day of the week, yes I would. And I do.
Without any reference tone or note? Then i'll come within a quarter tone, wich, I grant you, is still off, but not by much at all, and virtually inconsequential if you're gonna play by yourself.
If we're going for absolutes, even the tunebot is wrong because it says in the manual that getting each lug within +/- 1 hz of the target pitch is good enough.
A 2 hz differential between two tones can be heard, especially in the low to low-mid register.. But in the end, does it matter on a drum or collection of drums (wich a drumset is)? Unless you absolutely want to be uber- OCD about it, no, it reallistically doesn't.