It's a difficult one because, on one hand, you'll notice more of a difference with the speaker but if the amplifier doesn't have the guts to drive those new said speakers then you might not notice much of a difference and feel you have wasted your money.
In general, I prefer to buy cheaper speakers and drive them really well. It constantly surprises me how far budget speakers can be pushed and speakers always respond to power.
Personally, i'd drop the budget of the speakers even with a 1k amp, 5-600 quid would be the sweat spot. I'd look at the Spektor range, still a very nice step up over the mercury and rather than Denon look at Yamaha its a much better match. As the Dalis are a very smooth and rich sounding speaker and the Yamaha will counter that with a bit more zip to make them come alive a little more.
Edit, I don't mean to be rude saying "budget" as I know its still a good chunk of change, its just what the market segment refers to them as.