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. 

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   Fist off , I’m a strong advocate of premium NOS driver tubes . Yes they are expensive, but they’re  a long living tube and worth the investment. I must disagree with the “ JJ “ crowd , as my Rogue pre amp came with JJ 12AU7’s . I tried numerous tubes , with RCA being the best domestic and Telefunkens bettering everything I tried . I haven’t owned your particular amp , but I’d speculate that in a 3 driver tube configuration,  your middle tube will influence the sound significantly more than the other 2. That’s how it works in amps like  Bob Latinos. The sound is all based on system synergy , room character , and your taste . So you have driver tubes , power tubes and a reciter tube . Your driver tube(s) will have the biggest change in tonality . Your power tubes will matter in accordance with what speaker you have . Now I have about 10 different driver tubes , Including RCA, Seimens silver plate and CCa, Tekefunken Tektronix, G73-R, and standard, PHILLIPS, EI , and I can keep going . Now as for power tubes I have new production JJ, Gold Lion, Tung Sol and the Psvane Ti. NOS power tubes GEC KT88 and KT 66, Tungsol 6550 3 hole , and SED Winged C . And about 6-7 reciter tubes , mostly NOS . So here’s my recommendation. Driver tubes on a budget , Tungsram NOS , Or Tesla ( nos ) . Telefunkens are the best . I’ve tried nos Mullard and it’s like throwing a blanket over my speakers .  The sonic signature of all drives tubes is perfectly described on Brent Jesses site. Then for power tubes,  new production Gold Lions are the overall best at providing a flat response  and still provide good bass . In my system they are only bettered overall by KT -120’s and SED Winged C’s . The higher level Psvane Ti’s are too bright . Now with your rectifier tube , it’s important to see what your amp can accommodate. Mine will handle many . I have all of the good ones except a NOS GE 274b. I prefer a 5U4GB over the higher voltage 5AR4, as it smooths  out my system . So for you a modern Gold Lion or a vintage RCA or TungSol coke bottle if it’s compatible with your amp . About 15 years ago, when I got back into tubes, I had an Antique Sound Labs 30 Wpc integrated that had a SS power supply . I went with Tungsram drivers and Svetlana ( SED ) winged C EL34’s and the sound quality improvement was substantial. So junk any cheap Chinese tubes , skip over JJ’s and go to something that you can live with . Happy hunting , Mike . 
NOS tubes almost invariably beat out new production stuff....in both sound and life span.  There are a bunch of NOS brands that beat the snot out of JJ's, that's for sure.  I bought a Carver Crimson 275 that had the JJ 12 xx 7 signal tubes and I put in Telefunkens from the late 50's in and it was a revelation.  The I changed the Tungsol KT88, although the Tungsol are a pretty good power tube, with the Dawning Series Shuguang WEKT88 PLUS tubes.  I had an entirely different amp.  Just stunning.  Now in the past I wouldn't give you a nickel for any Chinese tube but I had tried the WE6SN7 PLUS Shuguang tubes in the Don Sachs pre at Don's suggestion and it was a stunning change.  I also prefer the NOS PHILLIPS  5U4G or Mullard GZ34 rectifier in my tube DAC.  Both from the 60's I believe.  In the DAC I also use NOS Siemens or Telefunken as the signal tubes.  The new production tubes just  have no comparison.  They are more expensive but well worth it in my opinion.  Brent Jesse and Andy at Vintage Tube Services are my go to guys.  And if what you order isn't the sound for you?  Just send them back.  But both these guys will put you in the right direction if you tell them the sound you're looking for.
Way back when Gold Aero were the best. That company got tubes from all over then tested and would only sell the ones that tested the best but good luck finding any..
I will agree that, once you’ve decided what tubes sound best to your ears, you’re probably going to keep your fav tubes in and not keep rolling them. 
In our Leben CS600, we rolled a bunch of stuff (including Russian Military Grade options) and finally decided that the GGLs sounded best for the type of music we listen to in that room (acoustic folk and acoustic country). 
That’s not to say it wasn’t fun hear how the different tubes sounded with different musical styles—especially thru Leben electronics. 
@milpai,exactly! And for him to put his name with the aforementioned new tubes says a lot itself. That he will go into something he doesn’t like and swallow his ego and say that he likes them. 
I’ll try those out for sure. Just not at his prices lol. I buy enough nos tubes off the guy