I am not aware of any commercial products intended for HiFi but I know of a guitar effects pedal, Mercury IV (harmonic boost), by Spaceman Effects that has a dial for generating even order harmonics. I imagine two could be used in dual-mono, which would meet the OPs requirements. They are on reverb for around $300. Something like the Switchcraft SC600 Dual Adapter Box would enable you to convert to 1/4" TS/TRS to RCA and back. I own this pedal and love it for guitar. The pedal can color or be transparent depending on the settings and gear on either end of it.
A Black Box to Solve Everything
Tubes vs. solid state?? There's a thread out right now about this exact question (there always is), but I didn't want to take it off subject. So my question is; is there a device that could go between the preamp and amp that has variable harmonic distortion capabilities? I know there are tube buffers, but they are not really addressing the point because they use line level signals and don't have the distortion of a good 300B tube being pushed beyond its limits. My thinking is that of a black box with switches and knobs to adjust harmonic distortion to mimic different tube types. It would also have to have a bypass switch so we could readily and quickly A/B the thing. Any ideas?