Yes, I also think that adding a USB interface before the converter could have a positive effect on the resulting sound. I found several independent threads on the Singxer SU-6 where at least 100 people agreed on this and had these feelings.
"Singxer SU-6 seems to have better clarity of musical lines and cleaner instrumentals. The musical lines are much more distinct. The music gets more transparent and relaxed. Also dynamics and bass is better. The bass is better controlled and a bit punchier. The piano sounds like a piano, you can tell the kind of flute, the tambourine sounds like it should. You can hear guitar strings being plucked."
The only thing I have to test is how it fits together with the RME ADI-2 DAC FS. Especially with the installed drivers, connecting apparently via a coaxial cable as I wrote in the first post (AES/EBU -> SPDIF coax RCA). I also have a Singxer SA-1 headphone amplifier connected to the RME ADI-2 DAC FS. And for me, I can write that with my HiFiMAN Arya Stealth headphones, the music sounds nicer or warmer with the Singxer SA-1 amplifier than with the headphone output of the RME ADI-2 DAC FS.
Singxer SA-1 is a high-performance, full-featured Class A/B headphone amplifier and preamp based on discrete, fully balanced circuitry. I would characterize the sound as very very clean and quiet (almost "dead silent"), smooth and warm like a creamy caress, detailed with a large amount of space and a rich soundscape, nice separation of individual musical instruments.
Anyway, I got that Singxer SU-6 second-hand, half a year still under warranty for a nicer price, so I'll give feedback when I try it in combination with the RME ADI-2 DAC FS.
At worst, I'll send it further out into the world and sell it to someone else.
