Looks to me like you are using a digital source at 44kHz sample rate, with an inadequate (possibly non-existent?) reconstruction filter. The jaggies are the quantization; the SEX acts as a moderate reconstruction filter but the solid state amp has a much more extended ultrasonic response. At a 44kHz sample rate, actually 44100 samples, the Nyquist frequency is 22050, so with a 22kHz signal, every 50 cycles the sampling goes in and out of phase with the sine wave - leading to an apparent fluctuating level after moderate filtering. A perfect reconstruction filter has an impulse response of infinite length, so it picks up information from a wider time frame to solve that problem.