I put the 2 Kappalite 3015s in the cabinet and it sounds great. Definitely louder than the Carvin PS-15s. With a 4 ohm load I can't get the fairly low wattage 200B past the 3rd mark, between 10 and 11 o'clock, without distorting. The Kappalite 3015s give me more volume with some nice low end at that volume setting. I think the amp and cabinet will work great for most of the gigs I do playing 60s music in mostly small venues. That is when we start playing again.
I hooked up my Fender Bassman 800 to the Kustom cabinet with the 3015s and it can get as loud as Ill ever need to be on stage. The 3015s sound really nice to me in the Kustom cabinet. They arent cheap but worth it.
I found the specs online for the Carvin PS-15s (not from Carvin though) and the VAS is higher for those than the Kappalite. The Kappalite is 153 liters and the PS-15 is 204 liters,