Disappointing 2.4ghz range on AC66U. What can I do to improve it?

I'm currently on John's Merlin fork 374.43_2-17E2j9527 after reading that it's the best firmware if you're looking for range improvement. It hasn't seemed almost any better than the standard Merlin build, although I was able to change my AC66U's regulatory domain so that I'd be able to have access to empty 5ghz channels. My 2.4ghz channel is also the one with the least amount of interference.

Adjusting...

Disappointing 2.4ghz range on AC66U. What can I do to improve it?