New sylvan boat with jbl ,mbb-118 sterio.all works fine except when you plug in mp3,then speakers sound tinny,no base,hollow sounding.

SOURCE: CLARION MARINE SPEAKER

Most covers hold themselves in place. You may have to pry it a little harder. It could have rust around the edges and will not come out easy.

SOURCE: Need users manual for JBL MBB 219S

http://www.manualnguide.com/brand/jbl/

hope this helps

SOURCE: bass tone problem

Both of your problem amps were made in the '70's or earlier. It is entirely possible that the coupling capacitors are failing. This is an issue for a service center whether it is caps or something else, like failing tubes.

The good news is that installing metalized polypropelene capacitors will make those classic amps sound incredible.

SOURCE: My son and his buddy recently installed an

If a boat is anything like a car, the amp's negative post should not be connected to the battery, but to the ground of the boat. In a car, the entire chassis is grounded to the negative terminal, so when you run a line from the positive terminal of the battery to the positive post on the amp, and a line from the negative post on the amp to the chassis it is a complete circuit. I highly recommend placing a fuse on the positive line from the battery to the amp. Wiring this way prevents the amp from blowing up your battery, as well as prevents the battery from blowing your amp. Hope it works.

SOURCE: Bad control board

sounds like a bad connection at the batteries. try looking for corrosion. If corrosion is found, clean and check for a tight connection on the connectors.