Neil,

I have one trap built as per some of your instructions ... and I have four
Auralex corner traps ... I just don't think I have enough trapping yet.

I agree.

the room "proper" is 19' x 19'

Besides the square shape, which requires more bass trapping than usual to
counter, that's also a relatively large room. I'd say you need at least a
dozen serious bass traps.

--Ethan