they dont have to be at the 4 furthest corners , you can bring em in closer into the shop , but placed higher up , say 8 feet with the 4 stacks facing each other at an angle for the sound to bounce lower rather than into the rafters
the serial number is usually stamped on the outside of the rear top panel of the speaker at the center , the stamping process stopped in the early to mid 80's ,
easy enough , 1st step , measure the dust cap , these are sold in various sizes ....Amazon , Ebay , Parts express
https://www.amazon.com/s?k=dust+caps+for+speakers&i=electronics&crid=SU2GAZMD31B3&sprefix=dust+caps+for+spea%2Celectronics%2C131&ref=nb_sb_ss_fb_1_18
you have too many subs and not enough speakers ,
4 stacks of 3 speakers at the 4 corners ,can work ,,,,,,,, 1 KP 1000 or KP 4000 + KP -3002 +KP 250 or KP 2002