It's really hard to know what will satisfy you and you didn't give a budget. (I'm not the one to give specific advice in any case.)
More what? Louder? What don't you like?
The better the existing sound the harder it is to get an improvement.

I don't know anything about the Solstice but it doesn't
look like a bad design...
With the sub I'd expect plenty of loudness and bass. You can usually EQ the higher frequencies. You might want to get a measurement mic and
REW to see how your speakers/room measure and what can be done with EQ, and maybe some room treatment.
Of course you're more limited with kits and you may not find usable-comparable independent measurements.
Transmission lines are also limiting but the box design (ported, sealed, transmission line, etc.) mostly affects the bass and that's much less important when you have a separate subwoofer.
If you go full-DIY, of course you can built anything and you can model bass performance with speaker design software. But overall on & off-axis frequency response isn't as easy to model/predict.