I'd suggest we open it up to everyone to answer. What's a good, reasonable affordable amp that can drive a wide variety of headphones well enough - not perfectly, but well enough to get a sense of what different headphones are like when adequately driven?
What a fantastic question. I’d have mentioned most everything you have, and perhaps add the Ferrum Oor, a very capable and dynamic little amp that packs a punch. There are used ones at the $1-1.5k range making that an excellent all-rounder in that price bracket.