89db speakers can use a fair amount of power. How many watts depends on the type of amp you want. My rule of thumb is, as an echo to a few posts above, a good amp should be able to double wattage between going from 8 to 4 ohms. This helps with reserves of power for transients. Speakers have big magnets to move.
This applies mostly to solid state A-B amps, which is mostly what I have had. Tube amps are a completely different conversation.
Some manufacturers under-report 8 ohm watts per channel so that they can show it doubling at 4. But get as close as you can, as well as the sound you like.