ok let me give it to you straight. most aftermarket speakers on a stock system are not gonna sound that much different unless you amp them. i dont know of any dual voice coil speakers that are aftermarket and if they were the speakers in the back are dual 4/10 ohm so yah it would be pretty much impossible to find a match. the gain should still work if you keep the stock headunit. so if you really want to have better sound you would want to somehow amp the speakers with more power because if you buy a nice set of speakers the rms will be much higher than the stockers and you will be underpowering them by a ton.
1994 SSE, 180 t-stat, gucci accented interior, suede headliner, eclipse avx5000 headunit, two 7.8" sunvisor screens, two 7" headrest screens, dvd player / tv tuner, infinity kappa perfect 5.1 components, infinity 12.1 perfect in the trunk, 2 rockford fosgate 350s powers, infinity refrence 7541a, 0 gauge audiobahn high flow wiring, duralast gold battery, sound dampened trunk and doors, xenon-white headlights, 2 15" and 1 20" red neons shine on the back doors, 2 3" and 6 12" blacklights lighting up the whole interior, red leds in the vents, red optx el neon pedals, red streetglow headlight leds, clear corners, pilot seven color angel eyes, blaze seven color underlighting kit, viper 791 xv.