Q1: No, you're wrong. The RH450 is @ 240 watts at 4 ohm & @ 180 at 8.
Best thing to do is forget everything you think you know about watts & just use your ears.
The RH450 is a loud amp, but the MarkBass cabs are pretty loud too.
Q2: I like the sound of MarkBass cabs (& amps too), but you might not. It's that personal preference thing
Best advice is to go & play through both (& others too) to see what you prefer.