Panel speakers sounded terrible to me, and even to my wife who noticed the muffled voices immediately. Quick investigation revealed that MMR has two cheap midrange panel speakers, hence no high end. Original MMs had one midrange and one tweeter which, while unbalanced side-to-side, at least provided a fuller frequency range overall. Aftermarket kits tend to use coaxial (2-way, midrange + tweeter) panel speakers.
I didn't want to wait on mail order, so I grabbed a pair of Pioneer TS-G1345R coaxial speakers on sale for $37.49 at Best Buy. They're 4 ohms, as are nearly all 5.25" car speakers I found, but MMR's speakers are 8 ohms. Due to the parallel wiring, MMR's two 8 ohm speakers provide 4 ohms of impedance. Simply replacing them with 4 ohm speakers would provide only 2 ohms of impedance, potentially stressing the amplifier. So I rewired the new speakers in series to provide 8 ohms of impedance.
New panel speakers sound phenomenal, but I felt the bass needed a boost to balance the bright & crisp high end. Alligator clipped a trusty ol' Polk subwoofer to the cabinet speaker, and now the game sounds basically perfect (aside from the startup bong and a few pesky audio bugs).
Thanks to Vince (TheKorn) for his advice on wiring the new speakers in series. His speaker replacement guide can be found here: http://www.thekorn.net/speakers/index.html