Out of those, probably the Beta is the most B-15-ish, but none of them are that close to the Ampeg thing (mostly because I think the tonestack in those amps is part of the magic). The Beta is definitely fat, though.
The Pork Loin can get fat. I would not classify the clipping in the PL as old school, but with the "curve" control all the way down it could get there. Maybe even close enough for you.
The Blueberry family of ODs are cool, and they sound old school in their own way, but I would not consider them "fat" as much as "thick". These kind of OD have a very distinct mid bump that you may love or hate. I happen to love it, but it's not for everyone.
Have not tried the Barbershop, but is supposed to be the most "transparent" of the bunch. Maybe a Barbershop + a Barber Linden EQ (it's modelled after the B-15 tonestack) could be the thing you are looking for?
From those not listed that I have experience with, the EAE Model feT is definitely old school sounding to me; for me "old school" meaning slightly farty with a quite "coarse" type of clipping. The kind of sound that modern band use when they want to sound 70s (think Rival Sons).