There might be a sort of basic/flat level of consciousness

That position is called pan-psychism. I don't think pan-psychism violates the rules of parsimony, but I also think once you find yourself asking how quantum vacuums or a baseball bats or water molecules subjectively feel, you need to back up a bit.

If this level exists, how do we know that destroying the brain interferes with it? What do you think?

Personally? I think that my qualia is that which separates reality from all the other hypothetical mathematical structures, and just leave it at that (so I evaluate consciousness from a strictly information-processing standpoint).. Well, that's the short version, I'd probably need to write a bit more for that to make sense.