What speakers to buy: Pioneer SP-BS22-LR, Definitive Technology 350 or Usher Audio S520

I'm far from an expert on these matters, but looking at the "Fanmusic" amp - I'd say upgrade to a newer receiver first. It doesn't even appear that your amp has digital inputs, which will limit the fidelity of the music to which you're listening unless you're listening to strictly analog media such as vinyl on a phono. If you listen to CDs or other digital music, you will want digital audio inputs - either coaxial or fiber optic - which provide an exactly translation of the music rather than an analog approximation of the music.

It sounds like you're a strictly 2 channel audio kinda guy, versus wanting an AV Receiver to control a home theater setup. In that case, I would go with the Def Tech speakers because they have the largest woofers and I've heard that they are well regarded for producing a full frequency range for stereo listening in a fairly compact package. If you were wanting an AV surround sound setup with multiple channels and a subwoofer I might suggest the Pioneers as better bang for your buck.