Well, 7 on the channel and 2 on the master is decently loud, but it shouldn't be feeding back that easily. Keep in mind, pulling the push-pull knobs out adds MORE GAIN. I won't tell you how to dial your amp, but... I'll tell you that pulling both the "fat" and "bright" knobs is, for most intents and purposes, way, way too much. Try one or the other, experiment with how each one affects the tone. To my ears, the fat knob just adds overall gain to the entire harmonic structure of the signal, whereas the bright knob just pushes the very top end up a bit. I never pull the bright knob, I find it to be too much.
Just for shits, try pushing both those knobs in and running your gain lower, around 4 or 5. Does it still feedback?
Also... what size is the room you're in? How close are you standing to the cab when you play?
The phase inverter tube has something to do with feeding the signal to the power supply. I'm not entirely sure of how it works, but I think it converts the signal back from DC to AC (don't quote me). It should be an AX7, swap it out and see if it helps.
Just for future reference, don't be so worried about changing tubes. Mesa amps are tough ****... unless you do something stupid like run the amp with tubes that are very obviously not supposed to be there (if they even fit the sockets, that is) or run the amp with no power tubes... you catch my drift. Mesa's #1 drive is to make amps that are reliable and sturdy. You can switch pre-amp tubes with the amp on if you want (you might blow the speakers from a "pop!' as the tube comes out, but the amp itself will be just fine.) AU7s, AT7s and AX7s are generally always interchangeable, at least in Mesa amps and at least to my knowledge and experience. They are all essentially the same tube, they just have differing levels of gain.