I don't have a better mind, but forum member ShotgunShane is experienced. He had both amps on hand and weighed in thus:
"I haven't done a direct a/b but it's really no contest. The mk2 is a much better amp. The clarity and transparency is amazing. It also has better bass control and better soundstage depth than the arrow. The arrow has a more upfront presentation that works well for hard rock and metal and a very well done bass boost. I also like the mk2 more than the O2.
I haven't tried the Dac section yet. I don't do much computer listening but will this weekend. The mk2 pretty much ends my searching for a highly resolving and transparent amp."
This is the thread it's from:
http://www.head-fi.org/t/602493/new-leckerton-uha-6s-mkii-portable-headphone-amplifier-announced-update-user-impressions-added-july-20th-2012