I wouldn't say a combo is trading "down", it's just different. There are some superb combos out there.
As has been said, separates are more flexible in many ways, but in some situations a combo could be a better option.
The Markbass Alain Caron Lite would be a good choice here (500+300 bi-amp combo, 36lbs), if you were absolutely set on a lightweight but loud combo, but I'd agree that getting a really nice powerful head would be a better option.