Using headunit rear amp channel

RonRizz

Sound better? not necessarily. Get more power to each driver?? more than likely yes.
Its a variable between the drivers impedance, and the cap/inductor used to filter the intended driver.
There are many combinations of driver/filter that can be used to present a 4 ohm load to your source unit, and it can get extremely complicated.
The easiest answer would be to get out your Multi-meter, and check the resistance of the woofer, and its filter separately, as well as your tweeter, and its filter separately. If both check out to be 4 ohms, you're good to go. You can hook them up as you described with no issues whatsoever.

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P99rs Head Unit--- Amps- 2 Pioneer Prs800d --- Zapco ST1000MX---Zapco ST-4XSQ---- SI BM mk5 Subs---SI TM65v2 Midbass--- SI M3 mids--- SI M25 xbl2 tweeters ----- 2016 MECA street class 3rd place at world finals