Hi, and welcome!
1. is it real quiet? -- Yes, except for microphonics if you tap or shake it. Some individual tubes are more sensitive than others.
2. is it stereo? -- Yes it is
3. will it supply 16 dB gain? -- No. In stock form, about 7dB; with the current source load or a choke load, maybe 13dB.
4. can it easily be powered off of an ac supply? -- No, it needs three independent supplies, two at 1.5v and one at 36v. I believe the supplies of the K-101A are not independent (they have a common ground), and I expect the "C" supply would not handle the necessary 100mA of current either.
One of the reasons this preamp is quiet is that it is not connected to the AC power and has no power line noise and no grounding issues. Note that a set of batteries lasts about 200 hours, and if you buy in bulk at Costco or similar you can get several sets for the price of the K-101A.
What signal level are you mixing? If you are at line level, the Foreplay (which normally has 10dB gain) can easily be configured for 20dB gain.