I use reatune in Reaper
Seems to be as accurate as any others i tried and has the bonus of having all Reapers pitch algos so that you can auto correct or manually correct it
I usually make a small pattern of 16 patterns, which I loop. I insert one synth of which I’m sure is in tune, and make it play a little melody. It doesn’t need to be a masterpiece, it’s just for reference. Then, I place the instrument which needs to be tuned next to it, playing the same melody. Keep changing the basenote untill they match. Finetune if needed. There you go!
Void Pointer, actually I did that too, but it wasn’t easy with cowbells and usually it takes me quite a while to do it. It’s much more convenient with a tuner.
If you’re trying to tune some bass to higher melodies elsewhere, try shifting the bass synth/samples up an octave or 2, tune them by ear, then shift down again.
Thats how ive always done it anyway.
I think having all your sounds totally 100% in tune can make things sound a bit dull so I’ll often treat tuning the same way I treat panning of percussion sounds - have everything sitting in a slightly different place (+/- 20 cents or so)