Ashdown Combo Help

Alright, so I only recently stepped into the area of "high end" bass amplification, and got a steal on an Ashdown ABM C410T-500 EVO II Combo from eBay. I'm really in love with it, volume and tone-wise it's exactly what I need. I've become interested in adding an extension cabinet - I love my combo's sound, I just need "more" of it.

First, I have to admit I'm scared witless when it comes to adding additional cabs. Wattage and ohmage and all that business intimidates me, especially when companies which produce products I love don't necessarily explain how they work in the best way. My combo amp is described by Ashdown as a "bi-amp", which I take to mean it only sends different signals to the four 10" speakers and the tweeter. Whatever. It's rated at 575 watts RMS, and I don't know what that means exactly. It doesn't say how much of that power goes into the speakers or how much goes into the tweeter or what the cabinet section is even rated at as far as ohmage goes. The output is labeled "minimum impedence 4 Ohms," which, fair enough.

So I assume that a reasonably-rated 8 or 4 Ohms cab is going to do well with this combo. I'm looking at an additional Ashdown ABM 4-10" cabinet. Now this is where I feel tricked - their newer 4 - 10" ABM cabs are rated 650 watts at 8 Ohms, while their older 4 - 10" cabs are rated 600 watts at 4 Ohms. The older one matches my combo as far as asthetics go, but the newer ones are obviously going to be easier to find, albeit more expensive than the used older model.

Edit: And what the hell does "900 watts programme" mean?!

So, assuming I can even find the older cab, which of them would go match best with my combo? My combo only has one speakon output for an additional cab, which is all I feel I need, but I'd love to know exactly what I'd be looking at as far how much power would go into each cab (the combo's and the extension) and how to safely go about rigging them together.

If anybody has any advice, or better yet experience with this stuff generally or Ashdown specifically, please help me out.