First of all congratulations on a very fine choice

I use one myself for live bass (it's the only piece of gear I've ever bought back).
The short answer is : plug instrument into Hi Z input - leave all switches out - plug Mic Level ouput to mixer - Enjoy
OK, Front panel left to right:
High Cut - push in to roll off highs. Basically an EQ preset.
Speaker to Input - toggles between front
Hi Z Input and rear
Speaker In. If you don't already know what this does leave it out for now.
Boost - Gain, volume, turn up until you see the pretty blue light flashing.
High Z Input - instrument input
Thru Out - sends an unaffected signal out to whatever you like (tuner, preamp, head, whatever) unless you push........................
Active To Thru - Thru Out is now affected by the boost and EQ.
Tone Button - push in to use the 6 preset tone curves. Leave out for a dry (not EQ'ed) signal.
Tone Knob - These are the tone banks. They are simply preset EQ curves. The last two are intended for use with acoustic guitar, they have a low end roll off. In reality you can use any or all of them. Use what works.
Back Panel:
Mic Level - Send this out to PA Mixer.
Line Level - send to rack gear (Rack Rig, Direct to Tape)
Phones -
Speaker In - You can connect the speaker output of a bass rig directly into this input. The toggle switch in the front turns this on.