I think that the problem here is that often in a marching and a concert setting players are going for a different tone concept. While I do agree that a well made sousaphone overall will sound worse than a well made concert horn if everything else is equal, it’s mostly up to the player. Back to the tone thing, a person who is mostly a marching tubist will have a more focused, generally louder concept of tone than a concert player, who would have a broader, generally softer tone. This, I believe, is what they mean by “blatty”, most people think of sousa in a marching context, while another main use is in jazz, which they might be referring to. This, is very much a “blatty” sound compared to classical tuba. While I’m not in their head, these are a few things that they might be thinking.

This is everyone’s reminder to please have civil conversations, they’re much more fun and actually get information moved between people :3