Well, today I use a Zähl HM-1.
At the time I had the RAAL HSA-1b and an SPL Phonitor X (the RAAL was better than the SPL with the HEDDphone).
That'll give you a bit over double the power of the DAVE into the HEDDphone (2,285 mW). I've not heard the XI Formula S/HEDDphone as a combination, so can't comment on that from a "sound" perspective. I believe the Formula S yields something like 104 dB SINAD (if you care about such things) at 300mW (90% of its power output capacity) into a 300 ohm load. Probably a bit worse into a lower impedance load, but you're also not likely to be running it at 90% power.