This is somewhat of an erroneous measurement.
A loudspeaker's impedance varies according to frequency, and the load presented to the amplifier will vary accordingly. This depends on several variables, number of winds in the coil, gauge of the wire, strength of the magnetic field, mechanical resistance of the driver, etc.
Just because the impedance matches with DC, doesn't mean that it will match with AC at the frequencies presented by the amplifier, unless all other variables are equal.
Will sound come out? Probably. Will it fry your amp? Probably not. However, the resistors are just going to dissipate power as heat and lower the efficiency of the woofer.
The re-cone solution is definitely the most ideal in terms of system compatibility. If you wanted to find the impedance of the factory driver, you'll need a sine wave generator and an oscilloscope.
Edit: Here's a document explaining how to measure the impedance of a speaker driver. Warning: trigonometry within.
http://www.cartchunk.org/audiotopics/ImpedMeasureFAQ.pdf
This is basically solid info. However, you do not necessarily need an oscilloscope and sine wave generator. There are inexpensive CDs that have sine wave tones on them and several regular voltmeters with true RMS capability to measure the impedance of the speaker at whatever frequency that you happen to be testing. It really isn't important, unless you are trying to plot an impedance curve anyway. In that case, due to the complex nature of impedance curves of speakers, a little knowledge could be a dangerous thing, since a nominally "8 Ohm" speaker driver may easily measure 30-40 Ohms near its free air resonance. Someone without experience may judge this speaker to be a much higher impedance than it really is.
A good rule of thumb for estimating a speaker's rated impedance is by just measuring it raw, which means with no signal, so the frequency is "0" or DC, and multiplying that number by about 1 1/3. So a 3.2 Ohm DC resistance will probably be about a 4 Ohm speaker. A 5.7 or 6.2 Ohm DC resistance will probably equate to a nominally 8 Ohm speaker, for example.
I imagine, not having looked into it, that the reason there are two sets of wires is because there are two coils, which allowed the system to be engineered to a price point with off the shelf components. You could easily test this by checking to see if there is continuity between the negative on one pair and the negative on the other, and also check the positive wires. If there is continuity, that means the amp is already mono, which I suspect it is, and you could just as easily use an 8 Ohm single voice coil speaker, with the other pair of wires taped off, or connected to the second set of terminals on the single voice coil, which wouldn't change the impedance presented to the amp, but it would give you something to do with the wires.
If the original dual voice coil speaker in the enclosure was rated at 7.5 Ohms, that is probably the rating of both factory speaker coils in parallel, so the amp can handle roughly an 8 Ohm load and put out about 25 Watts at that impedance. It is worth measuring a coil to see. No need to measure them both unless you're curious, since they will be essentially the same. If that is correct, assuming the amp is mono and the wires are already parallel, and you buy a sub with dual voice coils, you need to find one with either 16 Ohms per coil and wire them in parallel (+ to +, - to - on each side), or 4 Ohms per coil and wire them in series. That would entail using one pair of wires from the amp instead of both, and connecting one coil's + to the + wire, the other coil's - to the - wire, and then connecting the first coil's - to the second coil's + with a short jumper.
If you have any more questions, please post them here, and I'll try to check in every once in a while to see them.
, thanks for the writeups guys! I will try to contribute with this sub, take some pictures, and see how it goes, only $22 on ebay shipped
