Upgrade from MM651??

It looks to me like the next logical step is a DSP and to run those speakers active. This means you'll be taking the passive crossovers out and running the tweeters and woofers off 4 amplifier channels. When it already sounds great, changing out speakers might not give you what you're after. If you upgrade to "active" with a DSP that makes it much more practical to change the speakers and tune the system in a much more detailed way. The difficulty goes up quite a bit with this kind of upgrade, but it will really boost your system to the next level.

Alpine has a DSP, PXA-H800, and a controller for it as well. If you would have a shop do the work I recommend you keep an open mind to whatever DSP they have experience using. There are many good options, but not many shops are skilled enough to do a great job with a DSP they don't sell. I use JBL MS-8, and it does a lot of the tuning automatically. If you can install an amplifier you can install MS-8.

Those Polks are coaxials right? You could improve upon them with a component set. One popular speaker has been JBL MS-62C components. Supposedly they have pretty natural sounding tweeters and sound good with their stock passive crossovers.

As for DSP, is this head unit compatible with Alpine's Imprint processor? That could be a cheap way to go. If it's not, the next more affordable option is to get the Pioneer DEH-80PRS head unit. This one is active capable with automatic tuning capability. You can expect an instant improvement in sound with most components. Normally, the auto eq feature clears up the highs and mid-range pretty well. After that you can fine tune bass, time alignment, etc.