Sorry but this might seem really stupid, anyway i was wondering whether Combo amps with speaker output's can be plugged into cabs?
Isn't that the idea of a speaker output?
Fender Precision Bass
Fender Jazz Bass
1967 Fender Coronado Bass II
Warwick Star Bass
Squier Precision Bass TB

If you're also going to run the internal speaker, then an impedence change is necessary and they usually have a switch to change the input impedence to match the resulting impedence with the external cab added.

Or they may just disconnect the internal speaker automatically when a cab is plugged into the speaker out jack.

There are a number of ways to do it. I had a B-52 AT212 combo (seperated into a head and cab now) that easily allowed the internal speakers to be unplugged if desired, giving two output jacks which run in parallel.
Benford Strat x 2
Burton Strat
Burton Tele
Malden Karma
Lawrence Tele
Squier Deluxe Strat

Valve Junior/Bitmo Modded
Bugera V55HD/412H-BK
This sounds pretty sweet, now i just need to find a combo i like, probably the Bugera 6260.