IMO - if you like the Sony and it's capabilities and your only issue is the number of pre-outs then I would get a crossover or an EQ or a combination EQ/crossover. I would look into some other crossovers/EQs like Audiocontrol. After ten years of tweeking and tuning my sytems the EQ is the only way to adjust the right frequencies to get the best sound out of the system, regardless of the HU and components I am running. Most decks I have owned adjust only "tweeter, mid and bass" - not enough. External EQs offer adjustments to a dozen, 30 or even more bands (which may or may not be overkill in your case).

Anyways - I would definately go to some stores and search online for more info on both. Make sure you consider budget, car space, installation etc. If more pre-outs is all you need then an external crossover might cost the same as a new deck with additional pre-outs (depending on the deck of course) so check both out.

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ECLIPSE H/U - ZAPCO DIGITAL SERIES AMPS - DIAMOND AUDIO HEX/TDX SPEAKERS/SUB