This is my point. If 'SINAD versus pre out voltage' are comparable regardless of method change, X4800H is worse than X3800H below 1.3-1.4.
You mentioned 3 things to consider: SINAD at 2 V, SINAD versus pre out voltage, and SINAD at 5 W output
SINAD at 2 V: X4800H clear wins. It may be industry standard to measure at 2V. However, how many external amps need 2V?
SINAD versus pre out voltage: X3800H wins on lower end. This range is possibly in majority cases of normal listening. This is make me wondering how much is due to method change? Nobody knows unless you measure them using the same method. Further, if you don't convert the chart, you may not notice or think that this lower end performance is more or less important for your use case.
SINAD at 5 W output: X4800H and X3800H are comparable - 85~86.
Sounds like we are in agreement. "If", is the key word...