An audio mixer is useless to you.
A preamplifier might improve the sound quality. That would depend on how good it was. A tube preamp (or buffer) would certainly add warmth. A tube or solid state unit might drive the amp better so you would get better dynamic range. Lots of good used ones out there.
Since those are outdoor speakers they will tend to sound bright when used indoors. Tone controls would help you with that. The speakers will be your limiting factor so I wouldn't throw a lot into the rest of the system unless you are going to upgrade them.
If you are using them outdoors I would just use them as you are now.
A passive subwoofer would not be a good choice as you would have no way to adjust its volume in relation to the Polks. If you did use one it would likely connect to the Crown in parallel with the Polks. Some subs might have speaker outputs for the Polks which might remove some bass from them. That would allow them to play louder.
An active woofer would require its' own crossover/amp which would allow you to balance the level of the sub to the Polks.
Bass outdoors usually requires more power than indoors and will not cover as large an area since there are no wall.