I recently purchased a Markbass Mini CMD 121P. This is a combo amp, one 12 inch speaker powered by a 500 watt head. Now, the speaker itself can handle 400 watts. The head delivers 500W @ 4 ohm or 300W @ 8 ohm. Since it's an 8 ohm amp I'm currently getting 300 watts out of it, right? I wanted to add a 2x10 cab to it (Traveler 102P) but I don't know whether I should get the 4 or 8 ohm version. Also I don't know how I could know how much wattage is going to each cab (the 12 and the two 10's), or whether it's changeable. I apologize for my lack of knowledge! 
Here are the links to each amp's respective page:
Mini CMD:
[Invalid or Expired Link Removed]
Traveler 102P:
Invalid Link Removed

Here are the links to each amp's respective page:
Mini CMD:
[Invalid or Expired Link Removed]
Traveler 102P:
Invalid Link Removed