Audiobacon has provided some answers to this question, driving an Abyss Phi (which has low efficiency, but not as low as Susvara). In his review, the $15k Wells Audio Headtrip Reference beat DAVE Direct on every level including transparency. This is (primarily) a headphone amp that can output 50W into 8 ohms if needed.
On the same site, the near $6k Eleven XI Audio Formula S with additional power supply, also beat DAVE Direct overall, but not clear if that was on every level. As has been stated before, most external amps will lose transparency even if some users may prefer the overall end result.
Audiobacon typically uses expensive interconnects and power cords, so the cost of these can easily add one or two extra $k to the total cost.
My view is that if you have a DAVE then stop chasing after low efficiency headphones and save yourself a ton of money and space on your hifi rack. And instead put your energy into improving your source, as this will ultimately give you a better sound.