The most thoroughly tested and matched GL KT88's are those sold by The Tube Store, retail outlet of RAM Tube Works, the late Roger Modjeski's company. EL34's and 6550's, too. Also 12AX7's, and 6DJ8's, and many others.
Roger made sets for the amp's of others he liked: Mike Sander's Quicksilver and Ralph Karsten's Atma-Sphere for instance.