Inject headphone level into guitar amp input?

[Lectroid]

I have a Fender Mustang Micro, which is a plug-in guitar headphones amp with some decent digital effects.  For casual jams, it has all the effects I need.  So--is there some way to skip the pedals, and just route the FMM's headphone output straight into a (tube) guitar amp?  The manual says the headphone output levels are 30mW per channel.  The "Headphone Impedance Working Range" is quoted as "greater than 16Ωs".

I've read about effects loops in some posts here and they seem to be in the ballpark of what I want to do but I'm am not sure what's involved with interfacing two devices like this.   Can this even be done, and what's the best way to think about it? 

Is it as easy as just keeping the FMM's output volume really low when going into the guitar amp, and letting the guitar amp do all the amplification? Or is it important to match power levels more exactly, and probably impedances as well?  Is there a known approach for doing this?  I can see I might want to bring the two channels together into a mono signal with a pair of mixing resistors.  But there I run aground on the shoals of ignorance.

[DeepBias]

I would try an easy to build stereo-to-mono mixer, using only the tip/sleeve on the 1/4 mono jack (2 and 3 on the example below)

[Lectroid]

I would try an easy to build stereo-to-mono mixer, using only the tip/sleeve on the 1/4 mono jack (2 and 3 on the example below)

Thanks, that’s the one. It’s the simplest way to do stereo-to-mono that I know of.

[acheld]

I suggest trying the simple solution of plugging the headphone output into the effects "in" jack.   I don't think you'll hurt anything.  It may be that there will be an impedance mismatch, and then you'll need a bufffer.   But it might work just fine.   As you know, you can vary your output via the volume control on the amp.

Agree that the Fender micro is a good piece of kit once you have it dialed in to where you want it.  Not a big fan of the user interface, though.

[Lectroid]

@acheld,
Thanks for the suggestion.  If I had been smart enough to build  an effects loop, I’d definitely try that.   

I think it would work that way from what I’ve read.

I did try to plug it straight into my amp and it worked okay for a half-minute test as a proof of concept.  I stopped there because I don’t know if it’s safe and don’t want to harm the amp.

I’m hoping someone can explain the outlines of the problem so I know how to do it safely.