yes they are available. it's called F-14 or F-15 i think.

worst come to worst, get the F12, solder only the main and ground. You should get mono.

That impedence is rather trivial since the makers of the phones have obviously factored it's cut off point in. It just gives the amount of load resistance the phones give out. Another point is efficiency, which i'll explain below before i explain resistance, maybe Ijan can add in a thing or two in case i miss anything out.

Resistance of the cans themselves -

Well resistance indicates the amount of load a can poses to a source, like ijan said in another thread, this resistance is coupled with reactance, and non linear, but we'll leave that aside for a moment.

A good example is;

If you put the same amount of current and voltage (power in watts is the same), into 2 cans. One labeled A @ 75 ohms and another labeled B @ 300 ohms, the result will be like this (numbers only for education)

Can A = 60 dB attenuation
Can B = 80 dB attenuation

It means from an electrical standpoint, if both are fed the same signal, more power is lost on the one with a higher level of driver resistance.

Efficiency-

Let us get back to cans A and B, and use the same amount of power as described earlier. Let's say 100 miliwatts consumed. You must understand that even though can B's impedence is higher it does not mean that a 300 ohm can will sound 4 times softer (not factoring in Logarithmic curve) than a 75 ohm can. There is another factor. Efficiency!

What happens if can B with 300 ohms takes up all that power, YET! It turns 90 percent of that power into sound, and 10 percent wasted (heat, magnetic field loss or hysterisis) and the 75 ohm one is only 50 percent efficient, so you can say that even efficiency plays a bigger role in cans rather than just impedence.

ok, let's put it this way, (assume that both are fed the same amount of wattage regardless of load resistance, meaning both take up the same amount of power)

Can A 75 ohms = 10 percent efficiency
Can B 300 ohms = 100 percent efficiency

Which would be louder? obviously B, and when you combine the both ideas of resistance and efficiency, the conclusion is (if you don't like reading all my rubbish), that Efficiency and Resistance are a combo, even though higher resistance cans are harder to drive, they are in some cases much more efficient

Ok, now on to the resistance changing sound part

Resistance can change sound by the virtue of being a low pass filter, meaning it let's the lower frequencies pass and the higher frequencies are blocked. The theory behind it is, you can usually use A) Inductance or B) an RC filter (let's just assume B, because i can't calculate inductance worth a damn).

The formula for frequency cut off is Fc = 1/2pi x R x C

Fc being cut off, pi being apple pie, R being resistance (of the coil plus the new load) and C being (capacitance, parasitic or not)

The capacitance of the cans is quite hard to measure, maybe i'll get a new Fluke 189 to test since my 179 is only accurate to the nanofarad level. Although you should assume 100 pf for good measure. Just fill in the blanks and you'll have a cut off!

Adding resistance helps cut off unwanted higher order frequencies, leaving better or too much bass in some cases, it some cases it's a good thing, in others, it's even worse. So be wise.

Oh yeah, don't use those cheap resistors that have a tendency to drift (electromagnetic randomity and change into heat as the electrons bounce around), so buy mine i has RN55 and RN60Ds from Vishay Mind the unabashed sales pimping 

lets say my SR80 is assumed 75ohm. If i change the impedence to 750ohm, or perhaps 7500ohm, will it turn my Grado to a heavenly-sounded headphone?

and also if changes in impedence influence the sound signature as well, why headphones manufacturers don't just get it right before they ship the units out? Why we need to temper the impedence anyway?

Because the manufacturers each have their own sound signature, and i don't think everyone appreciates the frequency cut off because everyone has a different threshold of hearing, so they model around the theoretical maximums of human hearing. Most people aged 30 can only hear until 15khz, so why would grado make HPs that go up to 20? Because if some basket can hear until that and complains.....lol, point is everyone's hearing is different. I'm 20 and i can only hear until 17khz, i'm sure you're different too. And of course we haven't factored in drivers and stuff.

No it won't turn your grado into a heavenly phone lol, it depends on your hearing, to your ears, maybe anything higher than 18khz is perceived as junk that pollutes the sound, to others that can hear better, they might say you're taking their top end away.

Tell you what, the best solution is to test until which level you can hear by using high frequency sound files, and then use the formula given to determine a general RC filter parameter for your phones.

example, i want the cut off to be at 18khz

the formula would go 18000 = 1/ (2x22/7) x R x Capacitance of your HPs (you'll have to look the capacitance up), then just use basic algebra to get your desired R value, then take that value and minus it with the impedence of your phones (R cut off - R phones = R resistor).

Hope it helps

i was also wondering also, and what i think is, the good or bad is rely on the durabilities of the material, heat resistance and so on. but it's weired, scottiebaby who bought m COXIAL cable made from canare claims that there's significant improve over his old coxial cable.
but suppose coxial cable also transmiting digital signal. hence, i open up my mind which digital cable might also influence by conductance quality. and i think it's due to lower noise? (digital data got noise?)
anyone may enlighten me pls.

is it better if you can those 3.5mm splitters? One split to two, two split to four, four split to eight, eight to sixteen... and so on

huh? unless you make a toggle switch to disconnect one source and listen to another. Or easier, just unplug/plug the cable to the correct source when you need it.

You know why i'm asking such stupid question?
First of all i must thank my N90, which have no vibration function. 
When i'm in IEM, and when my phone rang, and if its my gf, and when i didn't pick up, there goes my head.
Now i'm thinking on connect my N90 and my source together sharing an IEM.
Or i might just sell my N90 and get another phone which got vibration function. 

You know why i'm asking such stupid question?
First of all i must thank my N90, which have no vibration function. 
When i'm in IEM, and when my phone rang, and if its my gf, and when i didn't pick up, there goes my head.
Now i'm thinking on connect my N90 and my source together sharing an IEM.
Or i might just sell my N90 and get another phone which got vibration function. 

how about when u on IEM, u connect ur N90 to ur MX5021.
and tune the  volume to 90%
=P

sell ur n90, get urself a 3310, it has the best vibration u know..
with the money left, buy urself a proper high-end headphone