No clue on headphone specs. Personally, I’ve never used the 3.5mm headphone jack. You never mentioned what 'phones you are using, which make a real difference on the need for a dedicated amp.
Volume control is via the BluOS app (desktop or mobile - it’s nice, by the way).
What I am setting up at my office: Node 2i + Lehmann Audio Rhinelander head amp. I recently added a new solid state head-amp to my head-fi table at home, so the Lehmann is heading into work.
But there are many, low-cost, good sounding amps. The Schiit offerings are popular, as well as the Alex Cavalli MassDrop amps.