je viens d'écrire à markbass et voici leur réponse très claire (ils m'ont répondu en 5 min!)
"Hi Pierre,
Thanks for writing and for choosing Markbass.
The head speaker outputs (one speakon output and one ¼” jack) are wired in parallel, and the IN/OUT connection of the cabinets are in parallel too. So you can connect each cabinet to the head’s speaker outputs using one speakon/speakon cable and one speakon/¼”jack speaker cable, or connect one cabinet to the head and then connect the two cabinets together with speakon speaker cable. Either way will have the same result, connecting the two 8ohm cabs the total impedance will be 4 ohms."