How much are you willing to spend? I have a Schertler A6 active magnetic pickup and it sounds fantastic, but it's definitely expensive. The Schertler with its little bonus mic is almost the perfect stage rig for me. ("Almost" because the separate volume controls are very difficult to use, but once it's dialed in it's magical.)
Duncan Mag Mic and Baggs M1A also have onboard preamps, and are significantly cheaper.
You can probably just replace the Takamine preamp, with either a stock or drop-in aftermarket replacement.