Do you make adjustments to VTF independent of VTA?Yes. I agree with everything Aoliviero and Nsgarch wrote about that. The VTF tuning we're discussing is so small there's little practical effect on VTA (SRA, actually, as NSgarch said). Nor are the sonic changes from VTF fine-tuning at all similar to those of SRA fine-tuning. These adjustments sound very different.
Do you think that the differences you note are due to the change in the force the stylus is applying to the groove or to effective changes in VTA (which must change with VTF changes)?No and no.
IMO the diffenences are due to the pressure that VTF applies to the cantilever/suspension interface. This is why excess VTF sounds so much like excess antiskating, as I mentioned above. They both apply too much external pressure and inhibit cantilever freedom.
The more pressure on the cantilever, the less free it is to respond. Its response to groove modulations will be slower and its peak amplitudes will be physically contstrained. The sonic result is smothered HF's and smothered dynamics and a general dullness or lifelessness.
OTOH, if VTF is too low, the arm and cartridge have insufficient inertia to resist moving when really big and slow transients (aka, bass notes) come along.
If the arm moves then some of the energy that should have deflected the cantilever relative to the magnets, doesn't. Result: softer bass with lower amplitudes.
Now where did I put that cartridge warming lamp? I actually suggested we use our Littlelite for that during winter months Neil, but Paul vetoed it for some reason. Oh well, another great tweak lost forever! I'm gonna design a tonearm with a built in heating coil and a thermostat. Should sound great!