A lot of good advice from the group here, although it doesn't necessarily square 100% with my experience, especially those who commented on impedance issues, tube life, and noise. I have two primary systems designed around the use of high and low-medium efficiency speakers, Altec 604 and Magnepan 3.6. I use tube amps from 1.5 wpc to 35 wpc with the Altecs, and tube and SS amps with the Maggies, tube preamp in both cases, and power output from the mid 50s to 120 wpc. The two SS amps I have range from 150 - 200 wpc. They all sound great because they're the keepers of the dozens of amps I've vetted.
The key is choose your speakers first. Then let the speakers drive the amp options. My tube amps don't care if they're running 4 ohm or 8 ohm loads, have had both, and the 4 ohm loads do not cause any issues with greater loading, heating, etc, with the tube amps, but yes, the SS amps run hotter with the 4 ohm load. As far as tube life, I run primarily old stock and NOS tubes and since I only listen seriously a couple hours a day, they seem to last forever. I can't recall the last time one of my tube amps was noisy for any reason. With the Maggies' lower sensitivity, tube amps are dead quiet. With the Altecs, there's a tiny bit of hum with the SET amp, and the rest are nearly silent.
Also great advice from those who said to avoid amps with proprietary or single source, or otherwise rare tubes. If an audition with your speaker choice is possible, then you'll be sure you're getting the best matchup for your audio investment. Have fun shopping!