Nice amp! congrats.
I also have an MK3 2500. As you say the master volume level is subjective. I think there are many factors that influence. Guitar. Place. Or how strong the drummer is. I don't play in any band now but sometimes I play with a drummer that will crush the battery very hard. I had to play the master volume at 8 with my LP standard.(He is grazy). If you do not use the loop connect a cable jack between the send and the return. More punch. Can you post a photo of the front panel?. Do you know what year it is? If you do not know, give us the serial number. Lately I only played with this amp.
. SORRY for english level
MARSHALL:TSL100/6100LM/JCM900 MKIII 50w/VM2466/JVM1H/
Lead 1960A/1965A/Vintage 1936/Lead 1936
Blackheart BH15H(moded) 7W-15W
Madamp J5MK1(moded 5W-10W)
Arial 4X12 60W