aberyclark OP
For even more naturalness/transparency
The First watt has an input impedance of 100kohm which anything can drive, and 20db of voltage gain, and your Zu Omen are 97dB!!!
You "could" very well lower even more the "electronic signature" count by flicking an unneeded preamp, and feeding your source be it dac or phono stage into a $49 Shiit Sys passive preamp with no electronic signature, and your sound could get even more transparent, if the gain is still enough.
https://www.schiit.com/products/sys
Or if your dac/cdp has level control try it direct into the J2, without using the Sys even.
Cheers George