A wooden cabinet can shake like a Parkinson's patient in an earthquake if you turn it loud enough. Anything can shake with enough force going back and forth. Even the moon.
The fucking moon. Over.
Your best bet is to just look at the speaker with a nice, bright flashlight (bright helps if you're looking through a grille, nice because shitty flashlights aren't worth the money saved) and look for movement. If you can't find any, kill all volume and look for some little detail that you'll be able to find - a certain shadow from the speaker onto itself, any kind of mark/tear/speck of dust, even the seam between the cone and cap - and look for it again while it's moving. More movement means more abnormality in the sound due to the speaker, and since we choose our speakers based on what they do to the sound this is a good thing for a good range of volumes. For the cabinet... Jesus Fucking Christ, it weighs more than the chair you're sitting in, if you can't tell when it's moving (hell, just put a bunch of screws on top of the thing in a pile and listen for rattle) then you have a problem.
Metal tone comes from a lot of things, and while this is one of them it won't make or break a sound as much as proper dialing and finding a place in the mix. Unless you've been recording at whisper volumes, you could very easily be only helping yourself by using less preamp gain - and with a poor cab, you don't want much tonal contribution anyway. I just saw the phrase 'great metal tone' and felt that a disclaimer was necessary, don't mean to insult your intelligence but I wouldn't expect worlds of difference.
Jeff