It's arrived and I am listening to it.
All I can say is Pro4S and SP540 sound quite different.. They are really completely different headphones despite both use same drivers.
Soundwise... I am not so sure if I can say Pro4S sounds better than SP540.. I am keep listening, but I am thinking that Koss engineers might had pushed this headphone too hard.
comfort and build quality, Pro4S is better on both regards. Due to being a studio headphone, it squeezes a head harder yet it is not uncomfortable level. There is enough space and extension unlike SP540 so it will probably fit everyone's head. The whole cups are made of aluminum, and plastic parts are non-rubberized. The padding of earpads is also slightly different from SP540 (slightly less)
One thing I hate about this headphone is coiled cable, and no straight cable is provided. I am currently using the cable came with SP540.
Here is a crappy picture.
