Have had my VTA ST-70 since 2013 running the same Gold Lion KT-66's. From reading this forum I was inclined to get a a new quad set since their age and the Holidays are coming up and people will be over listening. HOLY SMOKES!! My amp has "woken up". Streaming Jazz-on radio from Germany. At the same level on my Parasound Halo, I am now louder, bass is "deeper" for lack of a better term and overall detail is really sweet with the new quads. Not sure how to explaing it (Bob would know for sure) but for thosie having older sets of tubes, trust me, get some new ones.
Not just because of what I am hearing but highly recommend Vacuumtubevalues.com if you need some valves. Good price and avaiability along with quick delivery. Matched quad set and burn in for 24 hrs. 90 day warranty. Check them out.
Everyone have a Happy Thanksgiving and I am grateful for this wonderful hobby and everyone who posts so I can continue to learn.