For a boomy room, what panels will reduce mid-bass

You don't need panels, you need ASC Tube Traps. Go to www.acousticalsciences.com

Acoustical Sciences Corporation is the original and the best room treatment. They are most likely the most expensive too, but you get what you pay for.

I have been using ASC products for over 20 years and I am thoroughly satisfied.

Before getting panel or ASC tube traps, move your speakers away from the front wall and side wall. Toe in the speakers. It should help.

You might try some new and less boomy speakers.

I 2nd the tube traps, very effective and well worth the money!!