I have not tried the JJ 6L6GC power tubes. Not sure how good they are. Hopefully they will last longer than the 6CA7. I have a few JJ preamp tubes I pulled from a Carvin amp. Yes there is reason why I removed them. They seem a bit hotter than the Mesa branded tube. Not bad or anything but the Mark V is bright enough, unless you like that.
I wonder how the new Mullard 6L6GC's would compare to the EH6L6 or the Svetlana 6L6 or Tuns Sol 7581. I have the Svets 6L6, TS 7581 and the EH EL34. I also have the good tubes too (SED 6L6GC in Mark V, SED EL34 in RA100 and Gold Lion KT77 on reserve for when the SED's die).
I was so tempted to get the Eurotube Kits for the Mark V. The 6CA7 sounded incredible but I did not have luck with them lasting more than 2 weeks in use.
I spend a few days tube rolling three amps. Mark V, RA100, and Mark IV. What I found that worked great in the Mark V (as is the case with any tube roll or speaker selection, it is subjective)
V1, V3, V4= Preferred Series 7025 (selected Ruby HG tubes from the tube store, figured why not give them a try).
V2 = Mullard reissue
V5, V6 = CV4004
V7 = Sovtek LPS
R= EH 5U4G
V8-V11 SED =C= 6L6GC. The Tung Sol 7581 or the Svetlana 6L6GC almost sound very close but on the brittle side.
What I liked about the combination I ended up with is much better definition in CH3 MKIV mode, CH2 Crunch is intense. Actually all modes sound great in all three channels. One thing to consider, my tube roll exploited the best characteristics of the SED =C= tubes. It may be different with other power tubes.
I hope you find something that works at a reasonable cost.