returning goods bought over internet?

I recently bought a behringer pmp 1000 over an internet website, upon receiving this i discovered it wasn't as powerful as i was first led to believe, i paid £25 to return the item which i was going to replace for the next one up. After a week i Still had no Response from seller so decided to get in touch and get the original back as i need it for work, i again tried it out in my home studio and still was not happy with power. What are my rights on returning this product
 
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