Every 5998A that I've seen is simply a 6080/6AS7, not bad but not a real 5998 either. Also, FWIW all 5998s were made by TS, there are just many rebrands out there so no need to pay for the TS name if someone is trying to get a premium. Also, the 421A is just a 5998, but tests and listening seem to indicate that they are the lowest noise versions. Still, I wouldn't pay a super premium for a 421A either.