Loud is subjective for sure...
My Antelope can be a "safe loud" at maximum out, with 250ohm and sounds really nice, while the Grace901 and UX8 and Behringer HA. would be ear damage and are 50% volume with 250ohm with plenty of Loud volume to spare.
The weakest Ive had are the USB powered UX2 which is one reason why I bought the Behringer HA, in addition to having extra ports.
I think most users, at least myself, are used to the closed back low impedance sets volume and then get high impedance headphones to "upgrade" and notice the drop in volume...or as the OP says "they have to have the volume max'd out."
HA for $27 is fed by the Headphone Out of interface,?, using the Headphone Output of the MOTU 2 to feed the HA400 with a TRS cable per the youtube. (not the Main Line Out?)
After reading the 1 star reviews and 2 star reviews, bad jacks and hsssing noises....I cant say how good it is in build quality. Like a lot of cheap stuff "if you get one that is built well in the pile....you did ok"
The Behringer HA we bought was a rack mount , old thing but still works great, maybe one channel is bad...I cant recall.
$27 maybe worth a try. on the HA400...hope you get a "good one".