id try bracing the guitar in a way that the jack can not move and trying it. if feedback lessens then have the jack repaired/replaced.
this is good gear and you sound inexperienced. maybe you're letting the guitar pick up too much output from your amp.
try standing off to the side of the speaker instead of standing directly in front of it.
if the speaker is pointed directly at the guitar, try rotating it 90° away
1) It sometime happens if the cable pin is not getting proper connection with stuff inside the guitar hole. I had this prob with my Telecaster. The stuff that touches the pin inside guitar hole was kinda outta place.
2) Maybe the pickups and things near the guitars volume knobs are dirty. Use a cleaner.
cheap pickups, guitar pointed at the speaker of the amp which causes the pickups to router the sound throught the pickups over and over causing feeback. Most likely, cheap pickups.
A loose jack could cause feedback, but also if your pickup is too close to the strings or loose as well and if your gain setting is too high. Position in relation to the amp can have an affect as well, although it is more noticeable with a mic than a guitar pickup.
You could learn to play some Neil Young.....then the feedback would be desireable.