Naveenbnc's thread below discusses your first question.
About the rest, it is difficult not knowing room size and expectations and preferences for loudness. 25W on-up would be "safe" for "most people in most rooms most of the time" etc. Using Sandeepsasi's math in his thread at the bottom of the page , I get 25W @ 2.5m...which is both funny and good. And yes, were I to recommend a min sensitivity for an 8W amp, I'd start at about 93dB (which would be the same kind of safe for my position), but I really don't know it would work for you w/o knowing how big your space is and how loud you listen and what you expect for dynamics. People really are different.
Monacor published 85 in the Katana instruction manual (and rated it 4 ohm). By itself, modeling the spx-31m OB from scratch using Monacor data in halfspace looks closer to 88+ over most of its passband and I'm not seeing less than 6.4 ohms impedance where it runs, either. The problem comes when you see that HF driver paralleled with a lower load--the two paralleled woofers so it's likely there are places where impedance is 3.something ohms and thus their 4 ohm rating. Okay, well, looking at the other drivers (spm-205/8), yeah 3.6 ohms around 260Hz maybe. All this without losses or a whole system w/ filters, BTW. To get closer, you have to get impedance and phase data for each driver and to add the filter and source and all that.
It could be better than the data suggests and might work if your needs and spaces are less-demanding. Alternatively, the factory driver data may be optimistic and my estimates are off. What it tells you is that you might want to use the 4 ohm tap on your output transformer (if avail) and a power supply that can deliver current to get the most out of your 8W amp into these lower loads if you choose to do it.
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