Forgive me for my lack of knowledge with this one. If your amp has an effects loop, how do you know if it's series or parallel?
If you say so I tried making sense of it, but I'll trust your judgment.That looks to be a series loop http://schems.com/schematicheaven.net/manu/fender/vibro_king_manual.pdf
not sure what good parallel or even blend in an effects loop would do.
Considering that plugging into a single jack can also switch other things, I don't know if one can universally predict the results of doing so on one end of a loop......
I'm gonna try not to overstate this but this is the most succinct and therefore the best answer I have gotten to this question in literally years. I just tried it and am pleased to announced that I think you just saved me a grand or two in purchases.