90 db at 1 watt, at 1 meter from the speaker is how loud it will be. To carry that further:
2 watts will equal 93db
4 watts equal 96db
8 watts 99db
16 watts 102db
32 watts 105db
64 watts 108db
128 watts 111db
110db is THX level, 112db is ultra THX
You have to increase 10db to double the perceived volume.
90db is decently efficient and should be able to be driven by almost any amp over 5 watts.
Impedance is another story. Check to see what the nominal impedance is. And then see what the minimum impedance the speaker will draw is. You can then set the amp for whichever is best. If I have 8 ohm speakers that drop to 7 ohms, I would probably set the amp to 8 ohms. However, if the speakers drop to 5 ohms, you should use either 4 or 6 ohms taps. You will not hurt the amp by setting it to a lower impedance buy may cause problems the other way around. The problem here is that it will sound better the closer you can set the amp to match the speakers.
Finally, in this long note, many solid state amps will increase power output greatly as you decrease impedance (ie 100 watts @8ohms, 150 watts @4ohms). Many tube amps will put out the same power regardless of impedance.
