I've had the Xiang Sheng DAC-01A for a while now and was never struck with the quality of the 'tube' output, preferring the 'normal' output.
However even though the DAC is of commendable quality at its price point, the normal output does have that little bit of digital glare.

On investigation of the valve output/buffer stage I found that both input and output coupling capacitors were of Polyester dielectric, a definite no-no in my book, plus the valve was run at quite a low anode (plate) voltage, so certainly not in the best linear portion of the characteristic curves.

Also, just to provides that sods law element, the valve pin out was like nothing else, a 6N3P. So a little more complicated modification than I would have hoped for.
First off I replaced the output capacitors with some decent polypropylene's but that didn't make much of a difference on its own.

The valve socket is on a vertical daughter board so that had to come out and an alternative provided that could offer the different pinout.
The heater and HT power supplies were simplified in order to provide a greater voltage. The valve chosen was a PCC88.

The input capacitors were also changed for some polyprops, the yellow ones in the picture.



The end result was a healthy 90V across the valve plus a much cleaner 'tube' output without the digital glare of the normal output.

Result