I just bought a used Marshall AVT150 and a 4x12 cab and played at a practice session alongside the other guitarist who has a MG100 (50 watts smaller) and some older marshall cab with it. My buddy was on 3 maybe 4 volume, whereas I had to crank to like 8 or 9 and I could still hear him over me. Any thoughts on why? I'll check the setup when I go back tomorrow maybe the ohms on the back weren't set correctly but I'm just afraid maybe one of the speakers in my cab are blown or something. I don't know anything about amps/cabs so explain like I'm 5 years old. I appreciate the help.
play a note. stick your ear in the speaker. you will know if one is dead. not all volume pots are created equal, could be one is logarithmic the other could be linear.

tell him to get rid of the mg.
Ok let's assume that the speakers are fine, although watts don't directly lead to louder sound, I shouldn't have to be up around 7-9 just to even up to him.
First off, power amp ratings in guitar amps are made up.

Second, different speakers mean different sensitivity, which means a different volume of air moved using the same power, meaning some speakers produce a higher volume than other speakers with the same power.

Last but not least, different amps may well use different taper pots, so when you're on 6 you're using a third of the power the other guy's using, while you both use the full power when you crank the knob to max.