A good tip are speakers from Nubert. They're not well known outside Germany, but they make speakers with an amazing price/quality ratio. They only sell the speakers via their website or their own stores (direct marketing).
The best thing is: You have 4 weeks to test the speakers with your system at home. If you don't like them, you can send them back for free (EU-wide, also for UK) and you'll get your money back. They act this confident because they rarely get the speakers back; most people love them. I have a pair of their smallest speakers and what can i say, they sound great.
So, if you're not afraid of all the german text (or if you know how to use Babelfish), you may wanna take a look at the
Nubert shop.
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Will amplifier strength matter?
What do you mean with "strength"? Sure, the amp plays a part in how it all sounds, but a much smaller one than the speakers.
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my only other doubt is if a 5.1 or 7.1 system will sound better than just 2 speakers.
5.1 or 7.1 is a completely different deal than a stereo setup. I would only get 5.1 if you're into movies. For stereo music listening it has no benefit at all, on the contrary. 5.1 amps are built with 5.1 speakers in mind, the amp components for each channel take space and are often a bit small and feeble. Stereo amps can provide better quality with stereo speakers.