That looks like a well-featured deck. I have many Tascam models (DR-680, DR-2d, DR-60D), but not this one. Take some time to learn about it; I've found the interface becomes easy to understand with trial and error.
Assume you are recording from the internal mics? at any rate, make sure the mic input is set on low, and adjust the input volume on the side toggle switch as needed. There are possibly other menu settings to consider.
Read the manual. and I recommend that you check out taperssection.com; there is much info there and your deck is well covered in the Recording Gear section. good luck!
As noted above, a distorted ("brick-walled") recording is fairly impossible to repair.
(thanks NCH for the side track. And I commend you admins for paying such good attention to the forums!)