TRN is the OEM for FiiO UTWS1, so hardware difference is minimum. However, I do find UTWS1 to be better than the original BT-20S on the fact that UTWS1 has much less distortion and supports TWS+ mode (*also a lesser output volume, though I already find UTWS1 enough for what I used it for, mainly Smabat M1 pro). It was however speculated that TRN changed the later production BT-20S to be closer to UTWS1, though I can't confirm it myself
You can push UTWS1 loud enough for ST-10S Black Gold, but overall performance will suffer. I'll say you are probably only getting 70~80% of what ST-10S Black Gold is capable of. Not even BTR3K's 3.5mm has the needed power and control for ST-10S Black Gold to sound at its best.
Oh, that is too bad. I have a 150 ohm bud (pureson) and I think I know how it would sound on the UTWS1. The Pureson bud I have only sounds "right" when running on the Earstudio in 2X balance mode, the highs just don't come out otherwise.
Do you think the build quality on the UTWS is better than on the BT-20S? The pictures make it look more premium, but it comes out to almost double price locally, and I don't think I will be going Aliexpress after my last try. I might get it for my original ST-10 if it has less ground-noise.
Actually, speaking crazy, but how do you think it would do with an 8 ohm balanced armature driver?