15 ohm speakers yea or nay?

I just bought Falcon speakers that are 15 ohms. I have Harbeths that are 6 ohms. Both have the same sensitivity rating of 83db. I am using both with the same tube amps.

What has me scratchin’ my head is that the Falcons are much louder with the volume set the same on my preamp.

Any opinions on why a 15 ohm speaker would be that  much different than a 6 ohm speaker with the same sensitivity specs?

THX