Stew,
Since you're crazed enough to ask, the best primer I've seen on VTF and VTA adjustment by ear is at Lloyd Walker's site. Read this first:
http://www.walkeraudio.com/fine_tuning_your_turntable.htmAs LW points out, their are two levels of adjustment for VTA. Call them coarse and fine.
Coarse adjustments effect either tonal balance, bass vs. treble, or nothing. IME the degree of change varies from cartridge to cartridge. Coarse VTA changes affect tonal balance on my Shelter 901 but do little or nothing on my ZYX's.
The fine adjustment effects something much subtler, tightness and integration of notes in the TIME domain. (I'm not talking about image focus or tightness. Imaging happens in the SPATIAL domain, L vs. R, and is largely controlled by azimuth, not VTA.)
If you hear these time domain changes you'll hear them from anywhere. Being in the sweet spot is unnecessary (unlike imaging, which can only be evaluated from the sweet spot relative to your speakers). My partner hears VTA changes from two rooms away, especially if I goof it up!
When VTA/SRA is spot-on, each note gets as short and tight as possible. This is easier to hear with bass notes than treble, due to their longer wavelengths. The different frequencies that make up a sound are also integrated for maximum realism and impact. Getting drum, cymbal or piano hits right works. It is much easier to hear on good recordings, acoustic instruments and top caliber vocals than on poor recordings, amplified instruments or poor quality vocals.
It is harder to hear with unipivots than with more stable arms. It is harder to hear with conical styli than elliptical, and harder with elliptical than with fine line or micro-ridge. It is harder to hear in bloomy or less resolving systems than in tight and more resolving ones.
It's also harder to hear when dancing or singing along, so Newbee, Stltrains and Styx all have strong musical arguments on their side!