There must be a way to access them, why can't they be accessed

Well, there probably is a way to access them. The problem is that only the Empeg guys have access to the required documentation to do so, and they had to sign an NDA to get that. Mark doesn't work for Empeg so does not have the information. Even if he did, it'd be difficult to write a Hijack modification to access the treble/bass without revealing that information (hence breaking the terms of the NDA).

So, we either wait for the Empeg guys to include simple bass/treble controls in the player software, which isn't likely to happen anytime soon given that the unit is EOL and it isn't a bug fix, or Mark kludges it somehow.