XEmpire seemed to be doing very well before sold to Gamma. Since then new scenes are less frequent and less directors (now just Mason I think)
Mason herself sometimes directs for other Gamma owned sites which means less resources for XEmpire
I own some DarkX DVDs since I like to collect physical media too. Could be that all labels distributed by Mile High Media have outstanding video quality but when comparing to other studios on disc, DarkX is arguably the best. Upscaled on a Blu-ray player the DVD looks very close to native HD quality (technically it is shot in native HD or 4K)
Could be something as simple as only having 4 scenes per DVD and a running time around 2 hours so the video bitrate is still above average vs other studios who cram 4 hours worth of content and have trash compression so the DVD looks awful