What's the best (or is there a "best") DAC input to use?

You're welcome.  At the beginning USB was used as synchronous meaning D/A converter clock was derived from unstable computer data rate (to avoid loosing or gaining too many samples).  Async USB should be better.

Be careful with "better" transports.  Some of high end transports have fast transitions, in order of 5ns, making cable impedance match critical.  In such case using very short wire (that is not considered electrically "long") is pretty much impossible.  Rule of thumb says cable becomes transmission line (reflections) when its propagation is longer than 1/10 of transition time.  1/10 of 5ns would be 0.5ns equivalent to 10cm of wire (assuming 5ns/m) including connections inside of transport and the DAC.  On the other hand fast transitions reduce influence of electrical noise.

AES/EBU, a professional interface that was likely used for the master your music is derived from, will be best

USB is problematic in practice - besides the noise issues noted above, there can be ground loop problems

Toslink avoids the above, but...

then there is the issue of Redbook vs. Hi-Res

Schiit has a new product Eiter(?) and there is the Iso Regen, etc. and no doubt other manfs. have things (besides hyper-$$ cables) for USB

I would investigate the 2 boxes as both companies have return privileges

BTW, Firewire was good

the future is not always better

AES is best in my particular setup.

I have tried almost every connection for a DAC.  Of course, it depends on what the DAC will accept, but over the course of at least 100 DACs, this is my take on the best:

1.  I2S
2.  Firewire
3.  AES/EBU
4.  Coax with BNC
5.  Coax with RCA
6.  USB
7.  ATT Glass Fiber
8. Toslink