I would be more concerned with the efficiency.It appears that 3-4 ohms and up will work fine.Try to find a speaker with 89+ db efficiency and preferably a 8" woofer.The larger the woofer,the more power it will take to control it.
Tube amp and speaker impedence matching
I recently purchased a Conrad Johnson CAV-50 and am actively looking for the "right" speakers. I found a couple of speakers (GMA Europas for one - rated 4 ohm) I would like to purchase, but I am concerned about presenting my amp with an improper impedence match and causing damage. The CAV- 50 is rated "45 watts per channel from 30 Hz to 15 kHz, both channels driven into 4, 8, or 16 ohms". I'm pretty new to all this so any help will be appreciated.