Roxel AMP-150BT Amplifier for 2 Channel Wireless Stereo Audio Amplifier Receiver, Mini Hi-Fi Amp for Speakers up to 90W x 2, with Bass and Treble Control, Wireless Streaming for Passive Speakers: Amazon.co.uk:
Streaming Amplifiers — The Audio Co.
How to Make Wired Speakers Wireless
Bluesound Powernode (Gen 3) Wireless Music Streaming Amplifier – Ceiling Speaker World
How to make your wired speakers go wireless | TechRadar
The Sonos Amp is the best part of my audio setup | Tom's Guide