I think the Twin Valve is just excellent... small, light, powerful, not too expensive to revalve... I'm not exactly in love with the GP7 preamp but the amp sounds just great.
A 300w amp will be more powerful but IMHO not more powerful to the point to justify the much higher weight (at least 20-25kg) and the much higher cost to buy and mantain.
I think the Twin Valve is just a winner... but I would just hate having it in the combo version. I have an Ashdown 427 (very close to a V8, I think) and of course it sounds amazing but it's huge, ultra-heavy and costs easily 6 times the price of a Twin Valve (used vs used).