It would be unfair to say that "Martin's are boomy". However, Martin made their name in the bluegrass world (and somewhat in the folk world) with their Dreadnought models, particularly the D-28. This guitar was designed to be able to be heard in a band with multiple instruments. I think it is fair to say the D-28 is particularly strong on the bass end. That is not to say that they can't be played sweetly.
As a sometime performer and an owner of a D-28, I can tell you that played into a poorly placed microphone a D-28's bottom is way too strong. I believe most sound techs will tell you that they roll off some bass frequencies when miking a D-28.
Martin makes a wide range of guitars and there are smaller guitars that are more well-balanced than the D-28. So not all Martin guitars are boomy.
Roger in Baltimore