Strange noise/harmonics from reamping via the fx loop

[Aceshighhhh]

I've been doing some reamp testing with an external TS808 pedal and found that I've been getting some strange harmonics when using the effects loop (input 2 & output 2).

When generating a sine wave from my DAW and reamping through the FX loop (Pedal is OFF), I'm getting these strange even/odd-order harmonics:

Original sine wave:

Through FX loop:

Is it possible the signal is clipping somewhere?

The harmonics are still present even when the Output 2 level is turned down(not clipping input 2). I'm only using an FXLoop block at default settings in the preset. The harmonics are absent when reamping normally via USB.

I have the following I/O settings:

-Input 2 Mode: Left Only
-Output 2 Mode: Stereo
-USB/Digi Out Source: Output 1

My signal chain is DAW->USB->Axe Fx II->Output 2->TS808(Turned OFF)->Input2

The only other thing I can think of are the instrument cables connecting the TS808 to the Axe Fx, but I don't see how they could cause these harmonics.

 

[FractalAudio]

You're overdriving the pedal. The TS808's buffers are still active even when the pedal is bypassed and you're overdriving them.

 

[ETOLKIEN]

Ts808 is not true bypass, is buffered.
If you test a true bypass pedal in off position and take off the battery you will have the unaltered guitar signal (high impedance).
If you do the same with a buffered pedal you get no signal, an active circuit without current will not work. Put ond the battery again and you get a buffered signal (low impedance).
Yeah, I remember using a boss ds1 in off position between my guitar and a laptop line input for adapt impedances. Less than optimal but it works