Hi there!

Some research for future reference...

I've got a Marshall DSL15C (love it!)
BUT I know sometime I may have to upgrade to get loud enough to fill a venue. Which leads me to ask-

Those 8/16 ohm slots on the back, I seen in a review that they are fro speakers so would they go into a cabinet much like a head would? and would that improve volume and whatnot or have I got the system ALL wrong?

(I may have gotten the 40C when i got the amp but I really couldnt spare the extra cash!)

Yes, it would work just like a head.

However, more speakers generally makes a very negligible volume difference. The difference is more in bass response and whatnot.
Gibson SG Standard
Gibson Les Paul Traditional
Cort Explorer
Squire Standard Strat rebuilt with Fender USA parts
Squire Tele
Krank 1980
Orange Tiny Terror
Traynor YCV 50 Blue
Peavey Vypyr 75

Will fly for food. Call me Dylan
If you need to get that loud, mic the cab or speaker through the PA. I use a 50-watt tube amp, but I still mic it into the PA.
^^ this ^^ if you watch old Anthrax vids you'll see a Boogie combo on top of a 412 cab.
Playing on some new gear....review to follow
Ahh ok, thank you very much for the replies.
It'll be mic'd to the PA until I can afford a bigger wattage amp then!
Even if you can afford a 100-watt amp, I'd still recommend using a mic to get the audio from your guitar amp. Depending on the size of the venue, you may not be able to get loud enough to mix with the drummer, bass player, vocalist and everyone else. Besides, if a sound system is in use, the sound engineer will want to mic your cab, so he can control the mix. In the venue where I play, my 50-watt amp could probably cover, but I want to have a good mix with the others. My amp is turned up loud enough so I can hear myself play and the rest is handled by the audio engineer. And you don't need a fancy mic. Buy an SM-57 and a short mic stand and you're done. Make it easy on yourself and everyone else.