i also have a '77 B25B cab (loaded with Eminence Delta 15A drivers) . It has a selectable impedance switch. (4 or 16 ohms)
Ampeg modded that cab in 1973. It is deeper and widther than the old model.
You can see that on the '73 Ampeg catalog:
http://www.ampegv4.com/pdf/Ampeg%20A...-%201973-4.pdf
i use mine with a '73 V4 (6550's modded) and a Tokai '57 pb replica. it sounds thunderous!

better than with the SVT410HLF i had.
My only complaint is the weight and tall of that rig.
Do you think i can use my V4 with a SVT 210AV cab for small gig?
