Which tubes are the best?

Hi folks, I just got a Muzishare X7 integrated amp. It is beautifully made and sounds like it has a lot of promise. Even though I'm just breaking it in, I am thinking about tube changes. Why? It's just what I do. 
I'd like your opinion on the best replacement tubes, meaning smoothest, most "tubby" and mellow tubes. 
I won't be chasing NOS tubes. I'd like your experience with Gelenlex Gold Lion, TungSol, Mullard, Telefunken, or?,  etc. of current manufacture. The amp uses KT88, 12ax7, 12au7 and CR34 rectifier. 
Any thoughts about KT88 vs 6550 would be appreciated. I used to prefer 6550 way back when. 
Any information would be appreciated. Yes, I could go through all the old posts but I do have vision issues after four operations and that's why I'm asking straight out. 
Thanks in Advance. 
Back in the day, I used to do some serious tube rolling. The absolute mellowest, most laid back and detail rich tubes I ever found were some Mullards advertised in the back of Audio magazine. They had Magnus Organ printed on them. They were amazing. Unequalled. Ten years ago, I was cleaning out stuff and took a box of my small tubes to a local dealer. Some guy gave me $350 for them. Wish I still had them heh. 

Another vote for Psvane 12AU7 tubes. I have used Gold Lions. But the Psvanes bettered them. Both are better than the new Mullards in my system. I got mine from Viva Tubes. They have an online shop and have fast shipping.
Obviously Brent does not like new tubes. His business is to sell NOS :-)
I have not heard NOS in my system. But after hearing the difference between the new Mullards and the Psvane/Genalex Gold Lions, I am more than happy with the new tubes.
If « really » NOS tubes last between 6000 and 10000 hours and a new one only 2 or 3000, which is the more expense ice in the end ?

Sorry, just a novice here. Got a PrimaLuna amp that started to whiz a little so I went to get some new tubes. (This was just two weeks ago) After some research and reviews choose the "Natural Sound 6CA-7" which is a Shu Guang Perfect Pair. So while I was in the mood to spend some money I figured what the heck, I'll replace the 12AU7 as well. Landed on the Gold Lion Genalex ECC82/B749. Yay.
So put them in and loaded up some of my best music recordings. Well. Spent the next three days relistening to music that literally blew me away anew. I thought, dang, these Double Impacts ARE pretty good. Dang, this PrimaLuna IS pretty good. Everything went up to like double as good. Nothing changed but the tubes. And quiet, wow. But like I said, I'm no expert but I know what my ears hear.
My amp calls for 6550 output tubes so I auditioned them against a couple of other brands. I ended up preferring the Gold Lion KT88s by a significant margin so I bought them. And then I bought a back up set!

But that was on my power amp (CJ Premier 11a) - YMMV with other amps.