OK, let's start from the basics.
A bass speaker cab (whether it has one speaker or multiple speakers in it) needs quite a lot of electrical power fed into it - especially for bass frequencies to be pumped up to gig level.
The power to drive the speakers is provided by the power amp. The actual OUTPUT power (measured in watts) that the power amp produces depends upon the application it is intended for but can go all the way from small 1-10 watt power amps found in radios and practise amps, all the way through to 1000+ watt power amps used to power concert rigs.
All a power amp does is take a small voltage in, and crank it up to the output power that it is capable of providing. The power amp is usually designed so that it produces it's maximum power when it receives an input voltage of 0.75 volts rms (although this actual figure varies between professional and domestic applications). This voltage is called the 'line voltage'.
Now, when you ping a string, the pickup on your bass produces a very small AC voltage. This small voltage is nowhere near 0.75 v that the power amp needs to produce it's maximum power, in fact the voltage from the pickup is too puny to make the power amp produce any sound at all, so the signal from the bass is fed into a pre-amplifier (better known as a pre-amp) which will boost the puny output from the pick-ups from a teensy voltage up to line level. The pre-amp also does other clever stuff like all the tone shaping.
Again, don't be confused that an active bass may have a pre-amp - these are usually used to modify the tone and I can't think of any offhand that produce line level.
So, on your Mark Bass amp, you can use it with it's own power amp, but if you need more power you can also take a feed from the 'line out' and feed it into another power amp. Since the 250 watt power amp in the Mark Bass is connected to the pre-amp internally, the Mark Bass power amp will still be working and you should connect speakers to it to prevent damage, although modern amps are invariably protected against being run with no load, it's still a better idea to use it if you've got it.
This means that the second power amp (can be as powerful as you need it to be) will take it's signal from the Mark Bass with all the tone adjustment that you've applied and just amplify it to the maximum power output that it can provide. This means that you should also use cabs that are matched to the power of the amp so if the power amp produces 1000 watts RMS into 4 ohms, make sure that you get speakers whose impedance equals 4 ohms and that they're rated to cope with 1000 watts - you don't have to use an exact match, but it takes out the guesswork.
