“Nasal” Audio Note 3.1x Balanced.

I think you are hearing the combination of three "mellow"-sounding technical choices: non-oversampling DAC, 12au7 tubes plus paper-in-oil coupling caps.
These are all good choices and can sound awesome in the right system...however...if you have optimized your system for a previous, maybe brighter sounding DAC, your system may sound nasal with an AN DAC.
May I suggest the following "voicing" tweaks:
a-RCA clear top 12au7 tubes, very transparent and clear sound.
b- new coupling caps. The most transparent caps that I know are V-cap TFT, they sound like a clean window into the music.
Also, if by any chance your cables lean towards the mellow side, try Fusion Audio PCs (PCs DO make a huge contribution to system voicing) and Tara Labs Air 1 interconnects. These cables are not bright or tipped-up, they sound very transparent and neutral to my ears.
I hope this helps

Can other types of tubes be used in this dac? 12at7 ex.

Hello,

I had this Dac myself and sold it, without any doubts. An old DAC3 or a great DAC4 signature, are far better.
This 3.1x bal sound mechanic and crispy.

It seems that the new one is ok (MKII).

Ugo