How to stream Spotify from iPad to receiver?

You can use an Airport Express router or an Apple TV:

Play iTunes music on your speakers. Wirelessly. AirPlay takes the music from the iTunes library on your computer, iPhone, iPad, or iPod touch and sends it wirelessly to any stereo or speakers in your home. Simply connect your speakers to AirPort Express using an audio cable (sold separately). Then, using iTunes on your computer or the Music app on your iOS device, choose which speakers you want your music to play on. If you have more than one AirPort Express connected to speakers, you can play the same music from iTunes on your Mac or PC on all your speakers at the same time. Already have a Wi-Fi network in place? Use AirPort Express to add music streaming to its capabilities and create a whole-house musical experience.

Basically, play your track on Spotify, go full screen which reveals the sharing options, and from there you can pick your Airport Express which should be cabled to whatever AUX-in inputs your amp/receiver has.

The following pic' shows me doing something similar to an AppleTV (I don't own an airport express but the process is is essentially the same thing).

In fact for audio there isn't much difference between the an Apple TV and an Airport Express, and the choice of Apple TV or Airport Express is whether you want TV functions or Network functions as extra functionality over and above the audio. The only difference for audio is that the Apple TV has an optical out port (& HDMI), and the Airport Express has a 3.5mm Analogue/Optical output,

The iPhone version of Spotify doesn't have a full screen mode, but hit the i for info screen and it's next to the volume slider in the bottom right - easy to miss.