There are more impressions in the dedicated sources subforum as well. Seems like a top choice for a combo dac/amp around $150. FWIR probably a better headphone amp than the one that comes on the Zero, plus you can roll opamps.
Also about turning it up full volume: Check the dc offset of the headphone output, just to be safe. Mine somehow is way too high, i might've fried my opa2134 i can roll new opamps tomorrow and i'll measure it again.