The usual way to connect a line output to an instrument input is using a DI (typically passive) with an attenuation switch and a ground lift switch. That's comparatively advisable since instrument inputs tend to be rather sensitive to ground loop hum.
The Thump15A already has a 15" bass speaker. The typical bass amp will not really offer to move much more air. Its specified range starts at 32Hz and in the images and instructions I see nothing that would point towards a ported design (that uses a resonance port for lowering the specified bass response while giving higher rolloff below it at the cost of a more sluggish phase response).
A compact subwoofer tends to give more "oomph" at the cost of an even more sluggish phase response. A typical bass amp, however, is supposed to deliver a very precise onset at the cost of considerably coloring the signal and partly adding its own characteristics. That makes it an instrument amplifier and implies, among other things, that it will become locatable as the source of sound due to the characteristics of its nonharmonic distortions. Good for an instrument amp, bad for a PA component.
Mackie's dedicated subwoofer uses an 18" speaker. That can move more air than a 15" speaker without overly relying on resonance trickery. If you want to project a really big bass drum (like when working with electronic drums) or a church organ, this can make a difference compared to the unadorned Thump15A. For most other purposes, I am skeptical that using this functionality with a separate bass speaker cabinet is going to buy you much of an improvement. And I am particularly skeptical that using a bass guitar amp will make any reasonable sense as part of a setup in connection with a Thump15A.