Yes, I have put in 1 set of NOS tubes which are Seimens ecc83 from the 1960's which made for a more airy detailed presentation.I my try a full set of Telefunken's with the diamond emblem on the front at a later date.
I really like the sound of the Zesto Andros. It was ggod with the stock tubes, however, I prefer the sound with the 2 matched Seimens tubes.