Led reactive to bass preout from headunit
The 'step up' you need is more amplification of the line signal. Using a Darlington like TIP120 may be a solution in that direction, e.g. you can expect this.

Also, you've said nothing about what LEDs you're using. To flash the bog standard 20mA LEDs to music you don't need more than signal transistors (like 2N3904/2N3906), e.g. as seen here [the schematic symbol for the NPN transistor is wrong there by the way, something that none of the 30 or so commentators seem to have noticed], but you need to set up the right circuit; there is two-stage amplification there, much like a Darlington does. Transistors like TIP31 and TIP120 are power transistors capable of letting many amps through their collector-emitter.

To get good answers here you need to ask good questions.