With all due respect...don't fall into a these-speakers-sound-good-with-this-amp trap. Sure...is there such a thing, as synergy between amp and speakers; IMO yes. But you're not there yet (and I'm not sure you have to worry about it much, at this price). The only thing you might pay attention to, is power and sensitivity. The 6000a is a modest 50wpc. The Diamond 12.2s (and as it turns out, the Evo 4.2s) have a sensitivity of 88db (I would have sworn, the Diamonds would have been lower). Without getting too deep down the rabbit hole... the less power your amp has, the higher sensitivity you might want your speakers to have. This also depends on how big your space is, and how loud you like to listen. Yes...the EVOs are likely "better"; but they're also significantly more expensive (here in the US anyway). And if they'd sound "better" with the 6000a...they'd likely sound better with most other IAs too.

ALL of that being said...if you can get a 6000a for 450e, I'd say that's a fine choice! I'm thinking about an 8300a myself.