True, I have the same rendu and DAC as you do. I have had some issues keeping the Signature Rendu SE connected to the W4S DAC, which has required frequent power recycling in order to keep the USB handshake between the SE and the DAC. As a result, I have been in touch with both Sonore and W4S about this issue.
Sonore suggested that I try an ultradigital because the W4S DAC does have i2s capability, but W4S recommends using the USB input rather than i2s because they think it has better sound quality. It sounds as though you disagree with that assessment, but I have a few questions for you about using the ultradigital with the SE and 10th Anniversary DAC, if you don't mind:
(1) Any difficulty in maintaining a constant connection between the SE and the DAC when using the ultradigital?
(2) Have you been able to use hi-res files, e.g., native 256 DSD, to the same extent as you can with the USB connection?
(3) Does the choice of HDMI cable make a difference in sound quality? If so, which one did you decide to use?
(4) What preamp/amp or integrated are you using?
Not many folks seem to use this source and DAC combination, so I am reaching out. Also, knowing whether you have a more revealing amplification chain than do I will help me put your assessment of sound quality in some perspective. Thanks for any info you can provide.