If you have a computer, you should go for an audio interface to record with. Audio interfaces are used to take signals from mics and instruments and convert those signals to digital formats so you can mange them in a recording programme (daw) on the computer.

Audio interfaces have built in preamps in them, and they are fine for getting enough levels for bass. You can also record your synth with an audio interface.

If you find that the bass is recorded lower than the rest, turn down the synth tracks, so the tracks are at the same level, and go from there.

There are lots of good options here's a link http://www.musicradar.com/news/tech/13-of-the-best-budget-usb-audio-interfaces-570850