Keep in mind that Aune T1 is not tube amp - it is SS DAC with tube buffer + SS amp. And My aune T1 is recapped with ELNA SilmicII`s which made soundstage larger, boosted mids, tightened bass and tinybit smoothed highs.
Stock 6922EH tube is mainly good only for electronic atmospheric music/ EDM / Dubstep etc. and it has quite loose bass in stock Aune T1 configuration. Also 6922EH tubes seem to have problems with durability, maybe your tube was going bad and thats why some distortion. Anyway can`t hear any distortion with 6922EH tube + X1, no matter what genre.
And I`m using Aune T1 with my X1 @ +10dB gain setting. Atm listening to Animals as Leaders with Siemens E88CC and no distortion either. Instead hearing great tight bass and kick. But this tube has sometimes too large & holographic soundstage that doesn`t fit for genres like this so well as 69` Amperex 6DJ8 Orange Globe GAC, of course this is my subjective opinion and lot also depends on mastering and "metal" style.
Btw, just finished recabling my X1 with 3-metre cable (Van Damme Pro Grade Classic XKE Pro-patch + Neutrik jacks).
Measured stock cable impedance and result was very high 2.4 Ohm - that`s crazy! O_o.
New cable measures 0.9 Ohm. With new cable sound got tinybit cleaner and tighter.