I replace the EZ80 tube in my Border Patrol DAC SE USB+SPDIF with an EZ80 GE made in Great Britain. The tube was made for GE by Mullard. The improvements were significant. The DAC now sounds more dynamic and the high frequencies improved. My wife says its more lively.
Put Mullard BVA EZ81(Holland made) instead of stock Tungsram EZ80 - big improvement in tone and dynamics. EZ81 can always be used instead of EZ80 - the only difference EZ81 can handle higher plate voltage... Border DAC is stroke of genius.