I'm going to take a different tack on this. Since you said you have a mid-frequency bump on both pairs of speakers, yet it sounds great over headphones, I'm suspecting the vacuum-tube amplifier. Many tube amps do NOT have smooth frequency response and perhaps the bump is a product of non-linear response in the amplifier and or distortion. Did you try a solid-state amplifier instead of the tube amp?

Alan Lofft,
Axiom Resident Expert (Retired)