Hello.
There is no fixed minimal limit for an amplifier when driving speakers. You can drive your diamond's with a small 5W amp, but of course, not at full capacity. It's only important not to have an amplifier way over the max power of the speakers, since you can easily break them.
The Diamond 220 are recommended with at least a 25 watt amplifier, and no more than 100W, so the SMSL SA50 would work perfectly with them. If the SMSL SA50 have the looks you want, then they're going to sound perfectly well. Even the 30W TP-22 would be okay.
Speakers will not sound better with a more powerful amp, only have a higher sound level. Of course, a too small amp (less than 25watts for the 220s) will have lower sound levels. But what is important is not level, but quality.