It is included in both mods...but....The RWAK itself is really best suited to IEMs and lower impedance HPs by removing the resistance in the headphone out jack but still retaining the volume knob for use as is sans amp. The S mod is for amped setups and dispenses with the volume knob. They have 2 distinct functions, hence both offerings.
Each upgrades the DAC chip. I noticed a remarkable difference between the stock AK and the S mod, and I had such quick turn around on the work that I heard them nearly back to back. No chance to A/B obviously, but without getting into gushing adjectives for the sound I can only report I feel the RWAK-S is clearly, at least to me, a significant improvement over the stock AK both with cans and especially as a source for a 2 channel system. The sound is warmer, soundstage more open, improves dynamics and more air in the highs/mids (esp vocals and strings) and def improved bass bloom and bass definition. No regrets. I feel as if the AK should have been made to the specifications of the mod in the first place. I have an iMod that is more user friendly for me on a Mac, but sound improves from imod to AK to RWAK in a notable and impressive way. Really, I wish I had both the RWAK and the RWAK-S, but financially that's not sensible at all. I feel the price is fair for the mods and worth every penny. Working with Vinnie was pleasant, great service and I have plans to upgrade some of my gear to RWA components. I would imagine if you had a RWAK and wanted to go the S route, he would work with you on price. That's the feeling I get from dealing with him FWIW. If you are on the fence, I say go for it.