This is his reply regarding your question.
"Honestly, the differences is not as big as I anticipated.
that being said, 48960 does has slightly more details and slightly airier compare to original 2134, especially gave some sibilant to m40x. soundstage is wider just ever so slightly. 2134 has slightly deeper bass.
So I suspect the reasons they use 2134 in the X6 is because: 1.) doesnt makes much different. 2.) It is cheaper than 48960. 3) Deeper bass for general population.
At the same volume, low or high gain makes no differences, as DAC-x6 has rather low noise.
As Nwavguy said: a well designed circuit will not affected much by amp rolling, provided rolling with reasonable good amps, of course.
Amps that rolled (all bought from Aliexpress):
- NE5532 (USD1.59 for 20 pcs)
- 2604 (USD1.37 for 1pc)
- 48960 (USD4.25 each)
- MUSES8920D (coming soon, USD6 each)
Worth the USD4 investment? Yes if you critical listen."