Okay, so I removed the two rear screws and the volume pot. I tried to pull the bass boost slider out with tweezers and that didn't work because there's no way to get a grip on the slider. Eventually I removed the tiny slider peg by prying at it's base with a mini flat head screw driver.
Unfortunately, I still can't get the mother board assembly to slide out of the metal chassis. Not sure if I'm doing this correctly but I pushed inward on the volume pot axle. I applied what I consider a lot of pressure but nothing budged. Am I missing something here? Maybe the battery pack expanded over time and reduced the clearance in the chassis?
UPDATE: Ended up accidentally disconnecting one of the four battery wires from the motherboard. It was impossible to get back into the stupid little connector. Then when I tried to remove the wire connector the remaining wires disconnected. So I was left with a completely disconnected battery and no way to reattach it. The parts are too small for human eyes to fiddle with. I would need a jewelers magnifier lamp and steady hands to work such a miracle. Ended smashing the piece of crap in frustration. Felt good. My advice is don't disassemble your FiiO devices if you have one.