Well, having tried out the McCormack DNA-125 connected to the "Pre" (non-looped" output I have discovered a couple different things:
1) This Pre output is working as a true pre-amp output.
2) The TA-4A amp section is struggling in the upper mids and highs on my 4 Ohm Audio Physic Spark 3s. I think this might be an impedance issue, but maybe the Mc is just that much better in this frequency range. My Paradigm S1s didnt show such an extreme difference (they are 8 ohm). Bass is taught, clear, and lovely on either amp on these speakers.
3) The pre section of the TA-4A is nicely paired with the DNA amp. The phono stage sound is much improved in this setup. In addition to more neutral sound with a much improved high end, the channel separation is notably improved by a more discreetly laid out sound stage.
So, I think I've answered my own question!