David,
You can order the C4S at the same time as the kit, but you have to order the upgraded iron directly from Magnequest, and there may be some delays with that.
It wasn't clear if you meant to ask othher questions, and although my s.e.x. is not yet completed, there are some questions I would feel comfortable answering. And I'm sure there are others who will be glad to give you their opiniions as well, including experrience with different cans.
I plan to use mine with Sennheiser HD-600s, perhaps some Ety 4Ss and eventually some AKG K-601s.
HTH,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)