I have Aranov tubes on my Kef 4 speakers that are 4 Ohm. The Aranovs are noted for a good bottom end ....but I have a REL subwoofer also to help down low. Other than the Aranovs, many tube amps do have a specific setting for 4 ohms that will help match the load.
More important than the actual 4 ohm rating is the impeadence variance (i.e does the speaker drop to 2 ohms or even lower). Impeadence variance below 4 ohms can make things difficult for even the best amps. Efficency and design of speakers, and of course, power of the tube amp, also factor into the equation. -Lorne