JCX idea to isolate the problem by sending the tones separately on each channel (to avoid IMD at the DAC) and adding them before the Amp, to see if the problem is that the Amps can't handle ultrasonic stuff well seems pretty good. Here is a link that seems to talk about combining the stereo signal into mono before hitting the amp:
I was also trying to find an off-the-shelf in-line rca passive low-pass filter (0 to ~20 kHz) that you could plug between the DAC and the Amp... If the problem is that the Amp can't handle ultrasonics very well, then the filter might solve the issue....
EDIT: couldn't find one. Previous link was a high pass filter.
It maybe possible to build your own using a series of cascaded RC lowpass filters... Dunno the values, might depend on the impedance of the DAC output and Amp input. Someone else might chime in.
Edited by ultrabike - 12/12/13 at 12:10am