... I'm not getting the "top end" that I want; the recordings seem somewhat "flat". I don't see how it could be, but this wouldn't have anything to do with recording at the higher resolution would it? ...
Assuming the deck isn't defective, recording at higher bit depth and sampling rate isn't going to inherently make anything worse. Just the opposite, if anything. Overall, the sound quality you get about
so much else such as your mics, their placement, the room etc.
Stick with hi rez; you can't get it back later, and 24-bit will allow more precise edits even if the end product will only be 16/44.