I'm hoping I won't feel compelled to buy an SU-1 as I would really prefer not to add yet another box and signal cable between the NAA and DAC, as well as another power cable. In fact, I would really like to de-clutter!
I am not a fan of Holo's USB implementation. When I first took delivery of my Spring, I tried using it via "straight USB". I wasn't impressed. I put an Intona in the USB signal path and things improved. ...but nothing beat the SU-1 and the I2S connection.
I feel that the Holo Spring is a very good DAC "out of the box". However, you can really get it to punch over its weight class with an SU-1.