Now I'm not a technician, but IME deep lows mean cab volume, and cab volume means cab weight.
If you need to avoid that, then AFAIK, we're talking money.
Since 750 quid is way outside your budget, my guess for the time being is that keeping the Fusion is the way forward, unless I or hopefully someone else comes up with an idea that is better. The Barefaceds, Aguilars, Bergantinos and Vanderkleys on this planet all come with a price tag.
Could you do with a little MarkBass? They're voiced with personality (some hate them, but I love them) and very loud too. Also often a lot cheaper than aforementioned brands.
Edit: Discreet beat me to it.
Also, have you already been in Leamington to test some stuff? Mark should have enough in stock.