Pretty much any receiver with over 50W/ch will do. Since dB are on an exponential scale, most of the sound you will get out of those speakers will be within the first 10W. After that, the difference in output is relatively minor. For each doubling of power, you get a 3 db increase in sound output. I've seemed to collect a bunch of rather nice Dolby Pro surround receivers that are great for this application. These were replaced with newer 7.1 channel receivers for home theater use. I actually think most older receivers have better sound because they only had to pack 2 channels into an IC, whereas now they have to pack 7+ channels into their ICs to be able to supply all that other functionality for a reasonable cost. If you or friends don't have an old stereo or Dolby Pro surround receiver, you can probably find one at a garage sale or on eBay for cheap. There are still new ones too. Obviously, you don't need a 7.1 channel receiver for this application. Integrated amps, though typically more expensive, are also an option. Also Outdoor Speaker Depot sells small integrated amps for a decent price that will work well and have plenty of inputs.
BTW, on older receivers or integrated amps, you will typically have tape, CD, aux and phono inputs. The former three can all serve as independent sources via connection of your iPod, Squeezebox, computer soundcard, etc. via RCA cables. However, the phono input should be avoided, as it runs at a different impedance and has an RIAA equalization filter that will significantly alter the sound of one of these newer types of inputs. The rest should be fine for your use.
