i dont know about DAC quality but for Amp power sadly according to phonearena, the headphone output power on the 5T is only (0.47 Vrms) compared to (0.96 Vrms) on the original oneplus 5, (which means that you need a really efficient pair of headphones in order to enjoy that limited volume).
https://www.phonearena.com/reviews/OnePlus-5T-Review_id4454
scroll down to the Audio output section.
keep in mind that a HIFI smartphone like the LG V20 can output (1.78 Vrms) on high impedance device mode, (0.99 Vrms) on external audio device mode & (0.44 Vrms) on normal audio device mode respectively.
http://www.just-hifi.com/LG-V20-Sound-Quality_10182230-41.html
scroll down to the last post on that page