Dev,
The Halcro DM10 preamp has a fixed loading for MCs of 220 Ohms.
Setting VTA and VTF as well as Azimuth by ear, is a complex equation of achieving the best midrange, bass and treble with the most depth and transparency and greatest soundstage width and height.
In other words....trying to extract the 'magic' and maximum potential of each cartridge you may mount.
It is not easy and it may take many weeks (and it may change over time which is a real bummer)......but practice makes it easier.
From your question I get the impression that you have tried altering VTA and don't hear dramatic changes?
Unfortunately (or fortunately for all those who don't mess with these adjustments) Dev, the changes aren't dramatic or easily obvious but lots of practice and experimentation will eventually give you the confidence to proceed in a logical fashion.
Good luck.