W3NP Does it again, bringing you the very best in Ham Radio advice to the AM BB!!! That was a joke! Sound familiar??
Here is what I do with my modified Valiant when I run external audio to it (mostly I just use the D-104 tho).
The phone patch jack on the rear panel connects directly to the cathode of that 12AX7. I have had good success using a Radio Shack LO-Z balanced to HI-Z single ended transformer. They are built into an XLR connector body. There are two models with different gender ends. I just run a bal LO-Z cable from my audio rack to the RS xfmr and then a VERY short patch cable into that RCA jack for the patch, J3 I believe. Set the mic gain to a low setting and adjust your audio gear gain up for a good S/N ratio.
If you do go the TRS route, you may have to experiment with grounding. The 600 ohm feedpoint is NOT balanced...it is referenced to chassis ground.
Actually now that I think about it, I may have changed that patch jack J3 over to feed the grid of the 12AX7 right before the mic gain control. At any rate, it works FB with the RS transformer. The mic gain control on the Valiant does need to be set to a very low level.
I did the W6BM audio mods to my xmtr and am quite pleased with the audio response and distortion level. As I said, I mostly just feed the mic input with a D-104.