I did some playtime tests and here are the results:
10:17 16/44.1 FLAC (level 5)
09:11 24/96 FLAC (level 5)
FW 3.0, low gain, volume 60, Sennheiser CX-300 connected
The results seem to be OK.
But now the strange part.
First I was charging using the new 2.1A charger, charging current 0.82A (showed by the charging doctor), took little less than 4 hours to full charge.
For the second cycle I've used the older 2.1A charger, charging current 0.82A, took little less than 4 hours to full charge - this never happened before with this charger, it always took over 6 hours!
For the third cycle I've used again the older 2.1A charger, charging current 0.41A, took took 6,5 hours to full charge - why (only 0.41A)?
The charging current goes continually down, in the last third (guessed) of the charging process, to 0.01A in the end - next time I'll take notes, to be more exact.
Another thing - the X3 has a 3000mAh battery, if I'm generous 4x0,82 = 3.280Ah = 3280mAh, knowing there is a charging factor 1.4-1.6 (you have to deliver more charge, by that factor to fully charge) and taking the lowest, I get 2343mAh. And that calculation is very optimistic.
One explanation is, the charge doctor is very inaccurate or something is wrong with my X3.