A Black Box to Solve Everything

Tubes for guitar and tubes for hi fi have very different objectives. Audible distortion for a guitar amp is a desirable feature, not so for a hi fi amp. Two different worlds for the same technology.

Adding anything in the path of tube gear for hi fi runs the risk of masking the transparency that many hi fi tube lovers crave. It’s not uncommon to hear of SS hi fi amps that emulate the tonal balance of a tube amp (aka "tubey"), but they don’t necessarily have the same level of transparency that a good tube amp has.

Does your black box idea pertain to guitar, hi fi, or both?

I was thinking strictly for hi-fi.

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.

It isn't as simple as adding higher tube distortion. The Carver concept of transfer function actually works though, to  signmake a solid state amp sound like a matching tube amp, which is exactlly matching the electrical output of two amps. But I suspect the reverse won't work, matching a tube amp to a solid state one. Carver did the comparison matching one of his amps to I believe a Conrad Johnson. The listening panel agreed(after 2 tries I think). And I personally spoke to Gordon Holt and he confirmed that Carver suceeded. 

Don't use this one,

All the best,
Nonoise