I just ordered some Telefunken Tubes to upgrade the Art MPA II. I found a video on You Tube which showed how to mod the ART. The guy used Telefunken Tubes and played a A/B comparison demo. The Telefunken tubes sounded great...here's a link to that video.....
https://www.youtube.com/watch?v=IKFrkh1ZUdg
I found the Telefunken matched pair tubes at Reverb.com for $100 shipped...Here's a link...
https://reverb.com/item/638526-match...es-vacuum-tube
Yes you can buy cheaper tubes, but I wanted the Telefunken brand tubes...