Minimum recommended amplification requirements and amplifier matching

+1 lowrider,
Impedance, not sensitivity is a more important value.
Should your speakers dip below 4 ohms, you little amp is going to have a hard time keeping up.
When in doubt, call the manufacturer.
B

+2 @willemj , agree completely. 
I used a 300W/Ch/4 ohms with Monitor Audio Silver 8s with good results.

I tried a Line Magnetic with an older Gallo speaker 4 ohm 90 dB sensitivity.  It did not work well at all.  I concur with the other posts.  It’s the impedance not the sensitivity that matters.  Line Magnetic does not like low impedance speakers, but check with the distributor, Tone Imports.

I once drove a pair of monitor audio silvers with EKCO  integrated.  This is a very good tube amp indeed but with this pairing in triode the bass was very bloated.  Much better in UL mode.  I really wonder if the MA silvers aren't better off with SS gear. (Having said that I sold this to a gentleman who was very happy with this amp paired with guess what--Monitor Audio!)  Just be aware the pairing is critical.  It's often not the power that's the issue as much as the complexity of the load a given speaker presents to a tube amp--especially in triode or SE designs.  UL with a bit of negative feedback will be more predictable.

I like to run tube amps, and while 30 watts or so will make tunes on a speaker of 90 db, I think you will need more power. If it were my room, there would be no doubt. My speakers are 98 db and 16 ohms, and I really think 30 watts is about the minimum on them!

If you really want to work with tubes or even if you want your solid state to sound better, you're always better off not making the amp do something hard, like drive a difficult load. The amp will make less distortion!

To this end generally speaking, an 8 ohm speaker will sound better that 4 ohms and a 16 ohm speaker better than that, all other things being equal. This is simply because no matter the amp, it will make less distortion into higher impedances, and that is audible because the amp will sound smoother and more detailed.

So the takeaway here is if you like the tube amp- get a speaker that is more efficient/easier to drive.