So a lot of back and forth between the A90 Discrete and the X8's headphone amp and my final verdict is that the A90 Discrete produces a more detailed top end when the X8 is set to NOS/De-Emphasis (X9 signature clone mode) and bypassing the PEQ/DSP processing settings. Even on my HE1000se this results in a more engaging experience with all music that has a mixture of instrument layers, the "sparkle" (with no fatigue) as most would label it is noticeably present whereas it's slightly recessed on the X8's headphone amp, and in turn also the X9. Those who like that upper end signature will likely prefer the Luxsins, though my listening is on headphones that many still consider bright, even though I find them less so than my previous Hifimans.
This is with both amps volume matched by ear using the HE1000se, which meant A90 D at 68% and gain set to low, X8 on medium gain and at 72%.
Both X9 and X8 use the same OPA1612 + TPA6120A2 stages, to me they both sound almost identical baring the main DAC+DSP being different between them.
This comparison was using the X8 as the DAC. It could be that the extra power of the A90 D is scaling the HE1000se better than the /mere/ 4.8 watts of the X8, though I think this is less likely the case and more the fact that the A90 D is just a more resolving headphones amp than both X9 and X8, and in turn the K13 R2R with linear power supply.
Enabling the PEQ or sound controls in the X8's menu allows me to get close enough to the A90 D detail in the missing areas, though the detail layering on the Topping is still better handled.
With the HD600 I was unable to tell the same differences between HP amps, the Sennheiser Veil seems to mask away such details at the top end which I expected anyway.
I think the X8's breadth of features are wasted on me though, I'm using it solely as a USB DAC at this point lol, though the graphic EQ is nice.
Edit*
AC/DC's Long Way To The Top is a great example of what I am hearing, the electric guitar on the right channel feels electric in my entire right side, it's slightly recessed away via the X8's HP amp vs the A90 D.
I'm very impressed with the DAC side of things though, and don't actually miss the Warmer+Tubes much as a result. I think the DAC+DSP core is where the X8 truly shines.