[Elecraft] worse carrier rejection when KPA100 active?

[Amateurfunkstation TUHH]

I've built KPA100 for my K2. When doing two-tone test, I was
satisfied with results at 10 watts power (KPA100 in bypass), they
are shown on photo (1).

Then I increased the power to 11 watts, and what I saw wasn't just a
two-tone signal any more, see photo (2). As if carrier rejection
were not enough when running with KPA100.

So I decided to do a single tone test. While looking good at 10
watts (see photo 3), at 11 watt it looked like an AM signal with
small modulation depth (photo 4). I even played with carrier balance
pot on KSB2, and I was able to make it worse, hi. (Then I re-balanced
 it at 10 watts, with mic disconnected, using oscilloscope.)

Any ideas of possible reason?

Some comments to setup. KPA100 running into 100W dummy load.
Bias set to 400mA (2mV on .005 Ohm). Oscilloscope loosely coupled
via an isolated wire near the power resistor. Tones were generated from
PC (photo 5). A wattmeter showed (almost) no difference between
10 and 11 watt, so I hope kpa100's power meter is calibrated correctly
(photo 6). For every test, PC output was adjusted so that one ALC LED
was lighting (photo 7). No other changes were made when switching
between 10 and 11 watt.

(1) http://www.mydarc.de/dl2ipu/kpa100/two_tone_10_watt.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/two_tone_10_watt.jpg>
(2) http://www.mydarc.de/dl2ipu/kpa100/two_tone_11_watt.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/two_tone_11_watt.jpg>
(3) http://www.mydarc.de/dl2ipu/kpa100/one_tone_10_watt.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/one_tone_10_watt.jpg>
(4) http://www.mydarc.de/dl2ipu/kpa100/one_tone_11_watt.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/one_tone_11_watt.jpg>
(5) http://www.mydarc.de/dl2ipu/kpa100/tone_generator.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/tone_generator.jpg>
(6) http://www.mydarc.de/dl2ipu/kpa100/wattmeter.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/wattmeter.jpg>
(7) http://www.mydarc.de/dl2ipu/kpa100/k2_ALC_indication.jpg<https://mail.eu.cargill.com/exchweb/bin/redir.asp?URL=http://www.mydarc.de/dl2ipu/kpa100/k2_ALC_indication.jpg>

73! Oleg