Yaqin Integrated amp and tube rolling??

Well, the Mullard tubes are smooth and very pleasant sounding. I can imagine that if a system were even slightly in need of having any hardness or fizz tamed out these el34s would do the trick.
I had to try them, and probably over spent on the Upscale Audio cryo treatment and matching service.My advice if you are experimenting is to get basic matched sets off ebay until you find the sound you want. then splurge on Upscale's tubes when the ones you like best age after a year or so.
Going back to the Yaqin blue glass 6ca7's I get a less rounded sound with a tad more bite (More cabernet than merlot).
I would say if you prefer the sound of the amp in triode mode go for the EL34's. If you prefer Ultralinear mode stick with the 6ca7's, then try the KT77's.
Maybe I will be trying the Golden Lion KT77's this winter, to see if I like more "pop" to the sound, as Kevin at Upscale describes it. ( I'd prefer a wine analogy-can't abide pop!).
  
What a fun way to fine tune your system without getting a whole new amp :-)

A belated reply for other owners.
I have owned a Silk Audio MS-38 with Yaqin 6ca7 blue glass tubes for just over a year now.
It is a cosmetically modified version of the Yaqin MS20. I ran the supplied 5654 Chinese tubes for about 100 hours.
They did open up after about 40 hours, but the GE 5654's I replaced them with were immediately more refined. After burning in the sound was so sweet and emotionally involving I didn't want to  turn it off!  You could stop at that point, until the blue 6ca7s start to age.
I am now burning in a set of NOS Mullard 5654's and reissue Mullard cryo EL34s from Upscale Audio. Will report on my impressions soon....

Can I add on here and ask about the ms-20l and tube rolling options?

I had the MC-10L which used EL34's and 6N1 tubes. It appears yours uses 12AT7 which is probably good given you have a near unlimited opportunity for subs. I would search archives for 12AT7 subs for the pre tubes.

As for the EL34's My Yaquin came with Shuguang tubes which weren't bad. SED Wing C's are better. I ended up installing some Russian 6P3S-e's that were excellent (and cheap).

I no longer have my MC so the tube rolling has stopped.

Sorry, it's the MC-10T.

which model?