Danger...
Yeah, I'm thinking one (or two) of the (relatively weak) slim powered subs meant for tight spaces, under seats, etc. Some are only about 2.5" in depth, to mount on the rear seat back or the back of the hatch floor. Looking more for cargo space than boom boom, but definitely some natural bass and some ability to handle tha beats, in conjunction with some decent separates up front. Will be looking for an especially efficient and slim line amplifier.
I initially used an underseat sub in my Ford C-Max and it was awful. There was way too much midbass thanks to factory equalization, and by the time you lowered the crossover point to where it wasn't a problem, you'd gone lower than the tiny underseat box was capable of playing!
Ultimately my setup ended up like this: an Audison Bit Ten sound processor to remove factory EQ and substitute my own EQ (it also gave me line-out for a sub), an amp for the four speakers, an amp for the sub, two pairs of JL Audio component speakers (level 5 I think? nice ones), dynamat in the doors, and a Pioneer pancake sub & enclosure in the back.
Does it sound good? Yes, especially after a year of obsessive EQ tweaking. Does it sound good enough to justify my total expense? Absofreakinglutely not.
If I had to do it again, I'd buy the upgraded factory stereo option, and leave it the heck alone other than perhaps to add a powered sub like a Bazooka, Infinity BassLink, or JL Audio MicroSub.
All that said, I'm curious to see how deep the Bolt's trunk well is, and whether the Bose system includes a sub.