In my TAD-60, I just replaced my quad of Mullard XF2 EL34's with reissued Gold Lion KT77's. (I got a matched quad on eBay for about $150 shipped.) Honestly, I have been an NOS vintage tube user for years. However, these new Gold Lions are blowing me away; they are crystal clear and balanced from top to bottom. I was also using GE 6550's, but in my amp I found them to be too dark.