to clean up the TWIN-T CPO's high freq harmonic...
it first goes to a class D amp, then to a LOW PASS filter section of a 3-way speaker cross over network(L/C)
would also work the same for MUMBLE INPUT
the BEHRINGER UCA222 usb sound card has its own HARDWARE input monitor that could be used easily for monitoring your CW sidetone, in REAL TIME, with the 'cleaned up' twin - t osc via the WOOFER section of a 3-way cross over network