Re: 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.
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