On a budget, the Kenwood KFC-XP6903C speaker set is hard to beat (6x9 midbass and 3.5" coaxial set). I also like their KFC-X174 6.5" coaxials for the rear deck.
That being said, every "budget" 3.5" coaxial I tried in my 2018 had some really bad "harshness" at higher volumes. I would have to cut huge valleys in the 2khz - 4khz region to reduce that harshness. When I stepped up to a much "higher end" coaxial (the Illusion Audio C3CX), I no longer had to cut that huge valley in the 2khz - 4khz region. I'm guessing that it is some sort of distortion at higher volumes causing that "harshness" with the lower-level speakers. The Kenwood 3.5" speakers were better than the Infinity Reference that I tried first though (and the Kenwood 6x9 is awesome as well).
If you are willing to spend a little more, CDT Audio makes some great slim, carbon-fiber 6x9's that drop right in to these Challengers. They are great speakers. You can get the CL-69S version for like $100 when they are on sale (they are on-sale for $110 right now). I've measured them compared to the AudioFrog GS690 (much more expensive) and the CDT Audio 6x9 actually measured better in my car (in terms of un-EQ's frequency response).
I was so impressed with the CDT 6x9, that I tried their "Unity 8.0" speaker for my dash speakers. The "Unity 8.0" speakers are a 2.5" aluminum-cone wideband speaker with a frequency response spec of 200hz - 33khz! All from a single-cone speaker (no tweeter!). I love these little gems. They are
so "smooth" sounding. I've found that I simple do NOT need dedicated tweeters in this car. Since the dash speakers fire directly up into the windshield, the sound is reflected directly toward you, giving you "on-axis"-like performance from an off-axis dash speaker. So the Unity 8.0's give plenty of highs in the factory speaker location even without a tweeter. Since you only have a single speaker, there are no complexities related to crossovers - you get a huge frequency range (8 octaves, hence the "Unity 8.0" name) from a single driver.
However, I can't say how well they would work without a DSP to fully tune them properly. With only the Bass/Mid/Treble controls of the OEM headunit, I'm not sure how well they would sound.
Personally, I feel that a good DSP is tied with speakers for being the most important component in a modern system. These modern DSP's give you TONS of flexibility that allow you "tune" your system to your exact preferences. You get individual speaker level controls, crossover controls, time alignment, up to 31-bands-per-channel parametric EQ (so 248 bands of EQ compared to 3 with the OEM headunit), etc. A DSP can even make factory speakers sound
so much better.
However, once you start adding OEM integration devices (PAC AmpPro 4), aftermarket amps, aftermarket speakers, a nice DSP, sound-deadening, etc - you quickly fall down the car-audio rabbit hole and things get real expensive, real fast (ask me how I know!).
But, it is
so worth it if you love great-sounding audio. It's funny, I thought that the basic upgraded system in my old car (aftermarket speakers, small 50x4 amp, aftermarket head-unit and underseat sub) sounded so good - until I setup the system in my Challenger, complete with a Helix DSP.3. Puts the system in my old car to shame - BIG time.
Wow - I really rambled there - sorry for the super-long post.
