I have the Spider Valvle Mk II 112, great amp but I would like to use another amp head (Mesa Mini Rectifier 10/25W or an Orange Tiny Terror) and I wouldn't want to buy another cab.
Is it possible to plug another amp head (10/15/25 W max) in the Valve MkII cab or is the cab specially made for modelling only ?
Not at all. I've done it before with a Tiny Terror head, actually. Use the 8 ohm jack.