Hi Alan,
I'm not a slapper ( at least not a total slapper )
I only played finger style with a 5 string Yamaha PJ when I tested the 77, and it definitely handles the Low B and I think its sound is outstanding, it seems quite flat like a studio monitor so when comparing to other amps this has to be taken into account.
I love the sound of my Double Four and taken into context the size of the amp it is magic, The Session 77 is louder and would easily do well in an acoustic set and with a volume sensible drummer.
The only thing that stopped me from buying a 77 was the portability as I am constantly moving my practice amp around the house to avoid my wife and 6yr old son so I needed something very portable that can be hidden easily.
Cheers
Wade