This may be a well known fact but if you didn't know

[lampposteffect]

A technique my guitarist and I use to get the exact feedback you want is to use an EBO.
An ebo vibrates the string to give signal to the guitar while you can move your finger up and down the frets for the different notes.
It also has a "harmonic" switch that is supposed to give you the the...harmonic of the note, but it makes excellent feedback also.

This might help if you're completely isolating your guitar cab and can't get your guitar to feedback.