Question I am using a Combo 450 / Combo 750 / Combo Classic / Combo Staccato '51. I frequently need to pull out the head from the cab for separate (studio) use. When I put the head on top of the cabinet,  the speakON cable coming out from the cabinet is too short to be connected to the amp. Is it possible to connect the amp to the cabinet with a separate speakON cable

Yes, it is possible to connect the amp to one of the extra speakON connectors using another speakON cable.