Sensitivity 86 v 90

I would be concerned. Tube amps don't usually double in power from 8 to 4 ohms so moving to 8 ohm speakers will probably give you even less total power to work with. Especially if you're already almost max'ed out.

Tube amps usually put out the same power into 4 or 8 ohms so that might not make any difference.  But I am surprised that your tube amp has to work so hard to drive your 90db sensitivity speakers.  I have a much lower-powered tube amp driving Vandersteens (sensitivity 86 db) in a moderately large space to volumes higher than I want to listen to.  Perhaps the input level to your amp is too low?

If the ratings are reasonably accurate (a BIG “if”), the 8 ohm load of the lower efficiency speaker might make it a better match than the higher efficiency speaker.  I generally find that an easy load is more important when it comes to tube amps.

 Not knowing the size of your room, how loud you listen to music, etc., it is hard to speculate on whether you have enough power.  I generally expect 80 watts with 86 db/w speakers would be sufficient; at least I would not rule the amp out.

Look at the impedance dip. That is where things make a difference.

I had Salk Songtowers...easy to drive at 6 ohms and 88db efficiency with 30 watts. And great sounding speakers in their cost range. BUT it dipped into the 3 ohm range momentarily. There are subtle things that come to the fore with 93db speakers that I didn't so much with 88db. 

A lot of variables. It is correct that sensitivity on 4 ohm is normally measured with 2 volts and 2.83 volts on 8 ohms. These past couple of years, I see more and more speaker manufacturers labeling 4 ohm sensitivity @ 2.83 volts.

The amp is also a factor. Under normal conditions a good solid state amplifier can double its power output @ 4 ohms. With this scenario, you very well could hear a difference in a 4 ohm speaker vs 8.....(4db higher sensitivity plus 3db in power gain)

However, as mentioned earlier many tube amplifiers perform better at higher impedances and have separate taps for 4 and 8 ohms. In a good tube amplifier with reasonable power output and using 8 ohm taps, it is not likely that you would hear any appreciable difference. I believe that if you are not a head banger and have 40 watts per channel or more, you should be just fine with a tube amplifier and 86db 8 ohm speakers.  I hope this helps,  Tim