do I need component speakers if I just want a "decent" system?

RonRizz

Not the whole truth. Your components are going to play the same frequencies your coax will play.
The main difference besides the obvious (that they are seperated) is in the crossover it sends only specific frequencies to each driver, where in a coax the woofer gets full range, and the tweeter merely has a filter on it to cut out lower frequencies. this causes alot of overlap in the upper ranges.
both setups will play the full range of frequencies, components just do it better, and smarter.
A subwoofer adds richness to the sound, and both setups will benefit by adding one.
Lastly, I can tell you that if you do not intend to add a subwoofer, I would not buy those polks. although they sound good, they will not deliver in the lower frequencies. I have owned both the db and the mm series sets.

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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