First off, I should state that Mullard never produced a 6L6GC. This is not a reissue, but a brand new tube. The closest Mullard made in England would be the KT66 and KT77.
Physically, this Mullard 6L6GC tube is a kissing cousin the Tung-Sol 6L6GC-STR. Base, internals, micas and spring clips are virtually identical. You would assume they would sound identical, but they don’t. The clever folks at New Sensor must have been changing the coatings and materials while sticking with the established stamping and assembly.
Bass response is firm and the sound remains detailed. I put some of these in my ’59 Harmon Kardon and they were very pleasant to listen to. No ear fatigue and plenty of detail. They also performed well in my ’53 Fender Bandmaster. Lots of punch and some really nice bluesy sound when wound up.
Because of the short bottle configuration, these should be ideal in any amp, including high gain combos.