IMO I didn't feel the 15" Walkabout had that Portaflex sound. I had one and sold it quickly. In a loud rock setting, a B-15 won't cut it.
I have used B-15s extensively, mostly in recording situations, but some gigging too, over the past 30 years and I feel that the Mesa Walkabout 12 can easily cop that vibe. You have to watch the EQing because the WA really wants to put out the low end, where the B-15 is pretty mid loaded and very shy on any signal information much below 100hz.
I am not knocking the venerable B-15 in any way, they are great amps, and rightfully bass tone legends in the studio. But, IMO/IME, they are not great modern gigging amps because of their weight/dimensions and low power rating.
Great idea to have one in your arsenal though, once you get a steady working rig together.