Agree with this. Those Boss Katana's are quite loud, so you'll need a bit of punch behind you to keep up. A 250-300w combo would be ideal.
Although the Ashdown MAG's aren't the lightest around, they'll definitely keep up with 2 loud guitars and a drummist.
If you had kept the BlackStar Unity 500, that would have been ideal, as it's putting out something like 300w just as the combo (500w with the additional cab), and they're light.