Jeff, the whole power issue with the Krell 400cx and similar generation amps is very confusing and IMO, Krell is to blame.
The owners manual is clear and supports what Krell told you in your original post.
From the 400cx owners manual.
"Make sure the AC line is at least 15 amps for 300cx and 350Mc amplifiers; 400cx, 450Mcx, 700cx, and 750Mcx amplifiers should be operated from a dedicated 20-amp line."
The power cord unlike the previous generation FPB series is captive (sealed); so adding 20 amp service would entail making modifications to the power cord. Like cutting off the wall side and attaching the 20 amp plug. Who would want to do this on a multi thousand dollar brand new amp at the time? imo - Krell only did the 15 amp wall plug especially on the 400cx and up models for sales.
Pre CX series Krell amps had detachable cords. One could see the 20amp outlet design on the amp. The factory cord was a "dummy" power cord that had a 20 amp plug on the amp side and a 15 amp plug for the wall. All owners I know, including me, would just add in 20amp service and buy or make an aftermarket 20 amp power cord.