Ears are good way of doing things. I tend to do it all by ear really. That's how all the decent producers years ago would have done things.
Possibly - but "years ago" (I'd say "until the mid/late 80s"), decent producers usually worked with decent sound engineers - I mean, guys that were quite knowledgeable about acoustic, analog signal processing, AND music, and were able to properly record any acoustic instrument. Oh, and that could tell a good sounding preamp / EQ / mixing board / whatever from a lemon.
Looks like quite a lot of this knowledge has been lost during the late 80s / early 90s. Tape-op may not be such a fancy job, but you can learn a lot, at least if you use your ears and brain cells and have the opportunity to work at least once with a _real_ sound engineer.
One thing that amazed me was that so many so-called "engineers" didn't have a clue about acoustic and sound "physics" (sorry, lacking vocabulary here - pardon my French), never ever seemed to think about what they were doing and why, and just applied (often badly and/or inappropriatly) some "receipes" they didn't understand. Oh and yes - these guys constantly failed to notice that, however great it's automation was, the SSL boards were just plain lemons when it comes to tone (heck, even an old PA soundcraft 200B sounds better).
Not to claim I'm a good (nor even half decent) sound engineer myself BTW - was not my goal anyway...
Back on topic : ears are indeed a great tool. Still sometimes an external monitoring tool - frequency analyser, phase checker etc - can greatly help. Strange enough, the only guys I ever saw using these tools were the (very few) competent ones.