I think that when you see how most guys really find an improvement with just replacing the speakers, that tells you that the signal quality of the head unit and the factory amps are relatively clean. The problem with the verts, is they require alot more power than stock to be heard with the top down. Putting 50 watts rms to the factory speakers will blow them up in no time.
If you are looking for sound quality with the top down, your minimal option is replacing both speakers and amplification. Adding subs in the trunk will give your music that fullness that it so desperately lacks with the top down right now. If you own a vert and have a shaker 500, you know exactly how different and lacking the depth of the sound is with the top down.
2012 GT CS Ingot Silver Vert, Edelbrock E-force supercharger with CAI and 3.5 Pulley, Tremec T-56 Mag, Accufab 84.5, Centerforce DF, Billet Yoke aluminum driveshaft, FRPP 3.73 Rear, MRT Performance H-pipe, Bassani axle backs, JL Audio sound, Edelbrock strut tower brace, JPC adjustable UCA, UMI adjustable panhard bar, KW V3 coilovers, Steeda heavy duty front mounts, Steeda rear LCA's, 20" Steeda Spyders with Michelin Pilot Super Sports, Steeda Big Brake Kit ....... it just keeps going.