Using a resistor to do this is going to waste power.
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Not sure I understand the motivation to lower impedance as a whole. It's not the impedance that defines performance but, often, the performance decides the impedance. I'm gonna get flamed for oversimplifying.
That being said, if a resistor was placed parallel to the speaker and crossover, as a whole, it should not affect the crossover. If you were to reduce or raise impedance of any individual driver after the crossover, that would dramatically change the way the crossover functions.
Then, you have to consider wattage rating on the resistor.
You could add a large non-inductive resistor, with proper (high enough) wattage handling in parallel, with the speaker input. That would help lower the impedance. One of the results will be a lot of power wasted, as the resistor will be changing that power into heat, like mentioned above, due to the lower impedance.
One thing I don't understand is, why would anyone want to do this? It seems to be well accepted in tube amps, that a lower impedance speaker will typically degrade the amps sound. I can't ever remember hearing the opposite said. A lot of people also state the same can easily happen with solid state amps.
Another thing that wouldn't hurt if one was to do this, would be to have the equipment, and knowledge to run the equipment, to measure the speakers impedance after this change. It would also help a lot to understand how amps are designed, work, and how they will perform after this change.
One thing I'm just guessing is, you may think that the lower impedance speaker may be easier to drive, due to the lower number. But actually, the opposite is the case. A 4 ohm speaker is typically twice the load in comparison to an 8 ohm speaker. It sounds the opposite of how resistance works.
A zero ohm resistor would be a direct short. The same as connecting the amps outputs together with a piece of wire, or shorting them together with a screwdriver.
So, maybe this lower number sounds better, could be a reason for wanting to do this? That's just a guess on my part. A lot of speaker designers try to get the impedance number higher. The multiple drivers used in a 4 way speaker does lower the impedance, and this results in a harder load for the amp to drive.
In some cases, they use resistors in series with the drivers to help raise the impedance. Yes that raises the impedance ohm number, and typically should make it easier for the amp to drive.
Sometimes in a multi-driver system, they'll use higher impedance individual drivers, to get a higher impedance final number, in their design.
With all things being equal(specs, size, design etc.), a 4 ohm speaker is typically twice as hard on an amp, in comparison to an 8 ohm speaker. An 8 ohm speaker is typically twice as hard to drive, when compared to a 16 ohm speaker.
Here is a resistor calculator that may help. If you add an 8 ohm load to a 4 ohm load, you'll actually end up with 2.6667 ohms for the load. This also helps show you it helps to know some basic electronics, if your going to experiment this way.
A lot of us look for speakers with a higher impedance (ohms)
number, so it will be easier on the amp. I have some 4 ohm speakers that some of my amps can't drive very good. I'd like it if they were 8 ohms. It may have been too costly, or complicated for their designer to have made them an 8 ohm speaker, during their design.
Extra parts such as adding resistors (in series) to raise the impedance, can have trade-offs that result in the speaker not sounding good. That may be the case why there are a lot of 4 ohm speakers.
So, if it is a case of the lower the impedance number, makes it sound like it is easier to drive, the opposite is true. I hope this helps some.