Quote Originally Posted by hermanv
It is quite possible that the two drivers do not recieve the same signal, very often these designs feed bass to both drivers but mids to only one of them. Even if they are just wired in parallel, the tweeter will no longer be as loud as the mid/bass causing the voicing of the speaker to change by as much as 3 dB (possibly even more if the driver specs have changed) It is difficult to cover all the issues in a small space but let me give a simplistic example.

Suppose we have a series RL network configured as a low pass where the speaker is the R and a series inductor is the L. This common and simple first order network will allow only bass signals to reach the driver. The frequency at which the driver reaches 3dB is where R = 2 x Pi x F x L . As an example R = 8 Ohms, L equals 5mH, then F equals 254 Hz (i.e. the driver will get half power at 254 Hz.as frequency goes up less and less power will be deliverd to the driver) If we use a 4 Ohm driver in the identical circuit F becomes 127 Hz.

That is but one issue. The second issue is plain old watts. Power equals volts times amps. If we drive a speaker with the same voltage (i.e. the same volume setting on your amplifier) the 4 Ohm speaker will be twice as loud as the 8 Ohm speaker. This is because the current has doubled due to the resistor (speaker impedance) value being cut in half.

All of these things can be compensated for but they require changes in the crossover network. Speaker design is partially science and partially art. We can calculate the changes that need to be made to other components to compensate for a different driver but without measurement tools and a good ear, we may never be able to make the speaker sound good. Conversely it might actually sound better, but this is betting that the original designer did not do a good job, probably a poor bet.

There is no harm in simply trying it., Only you can decide if the end result needs fiurther work or is accepatble as is. The odds are just not in your favor.
i think you hit the nail on the head with that last paragraph. i think they sound bloody good as it is. and you may not believe me but in certain areas i feel that they actually excel my new mission m35i's and im quite content with how they sound. i realised that for a basis for comparison i could put both 8 ohm units in the one cabinet and have a listen (the originals were 8 ohm.) also, TOPSPEED, theres no need to pull your speakers apart as i have my original drivers (lovely looking things as well arent they. always covers off with my originals, but the new drivers arent as classy looking and also, the two 8s have tranlucent cones. looks tacky. the4 4s are ok though, just not a concave voice coil and no MISSION on the voice coil, obviously)