Sounds like a crazy idea eh? I know, but it does work. Let me explain: Ideally you should know the tone of your guitaramps in your head. If you know what they sound like before you play you will know what you can get out of them and how they will respond in any situation.
If you have a few guitaramps around plug in a radio into them and listen: Listen to how the controls respond while you listen to speech, change the settings and see what happens. Some amps may sound tinny, no matter what you do with the controls, others will change and give you a much better sound to the spoken word.
A guitar will respond differently, and your amps are for guitar, so really………
The radio-test is to give you in a nutshell what the general tone of your amp is like. If you have two almost, similar amps the differences may show up better when you listen to speech.
You can also do the test with music, but speech is quite good, as it is natural and not much has been added to the sound of what you hear.
What about plugging a mike into a guitaramp? You can do this, but the results are not great: You need to cranck the volume to get good results, not like with radio —radio will have its own volume control which works like another preamp, plenty of volume there, not the same with a microphone—-
Happy experimenting with the tone of your amp and hope to catch you soon again,