There are several options out there ranging from $20 to $400. I’m probably using the cheapest linear power supply with mine and can’t say if it sounds better but it definitely does not sound worse. I think any external PS is better than that cheap wall wart.
https://www.parts-express.com/power-supply-regulated-3-12-vdc-2a-6-way--120-536
This one is also a good reasonable option:
http://www.swagmanlab.com/Product_PSU?product_id=599
https://www.parts-express.com/power-supply-regulated-3-12-vdc-2a-6-way--120-536
This one is also a good reasonable option:
http://www.swagmanlab.com/Product_PSU?product_id=599