Since 3 weeks a Xduuo TA-20 Plus is in the house to give the Fiio M17 some tuby charakter. First impressions were ok, but the veiled highs were annoying. I missed to much details at the highs. So I changed the original ECC82 to JJ, Genalex or TAD tubes. JJ were better than the orinals, TAD very detailed and clear, but not tuby enough and Genalex Seemeilen to be the right compromise - warmer and good allrounder. The Xduuo sounded great with Grado gs3000e and Hifiman HE1000 Stealth.
But than, an Autrian Audio Composer arrived and it didn't sound better with the tubeamp than right out of the Fiio M17. It made fun with the Little Dot mk1 ( with Burson V7 Vivid and Muulard Tubres) because of it's punchy bass, but XDuuo was no advantage.
Goodluck I have ordered a 5Z4G , and that made a big difference - more than I expected from this tube! This tube was 30.-€ and changed this amp to another level.
The xDuuo now has clear bells, open violins, voices in front of you, warm wooden instruments, great piano, hard hitting drums and all that on a wide and deep stage - at least with the Composer. Love this combination!