Measure the resistance of your stock side dash speakers to get an idea about their impedance. With harman/jbl/infinity/levinson.... never assume. Different ohm rating is NOT the end all. A combination of speaker capability, efficacy, efficiency, and its impedance will determine whether it'll be close enough to work with the OE tuning, and power provided, for/from/by your vehicle. I'd test the center too.
Concerning the door, loss of volume level with higher impedance speakers depends on efficiency comparison of old vs new, and depends on honesty, or lack of honesty, concerning aftermarket speaker specifications, which are usually off by a mile.
Concerning the dash, considerably less power is needed to produce midrange and tweeter output when compared to the door woofer(midbassmidwoofer). So, you shouldn't be hearing much of a difference if afternarket dash side speakers were wired properly, with consideration to the signal coming from the OE/Infinity/Hyundai amplifier. You need to learn what frequency range is sent there and make sure your chosen speaker can play it!
Need to run a test tone sweep and mic to see what frequencies the door is getting and what the dash(each side and center) is getting. Too many practice shotgun speaker replacement which never leads to happiness. Center dash is probably another 3.5 widebander as automakers push soundstage and complexity vs simplicity. If so, more than like highs and not mids in the left/right side speakers. Test away.
At 1st, you wanted a replacement L/R dash speaker set. Now you/some are talking about amps/LOCs.... doors/centerdash..... Do you want to go down that rabbit hole?
If you were happy with the OE system, then the best best is a new dash speaker, or used from salvage yard/
ebay. BTW, Toyota also used a dash coaxial on their JBL system. If you can't find it from Hyundai, the maybe look at a few salvage yard Toyota's with JBL and see if you can make it fit. 86160-0WU50 usually found in various JBL equipped Toyotas and can be adapted to almost anywhere a 2.7-3.5" widebander were originally fitted. A few years back, I grabbed a pair from the local pick salvage for $10. Didn't sound any better/worse than the common overhyped pathetic sounding kicker/jbl/infinity/kenwood 3.5" coaxial many thought sounded great.
Learn about your OE infinity system prior to shotgun attempts are making it better.
Besides jbl/infinity, Memphis, MassiveAudio, Audison, CDT, NVX, Focal, Powerbass are some other brands offering 2-ohm speakers.