I used 5881WXTs in my Sherwood C300 mono integrated amp. The amp developed a strange problem - intermittent hum. I would turn it on and it would be fine for 5-10 minutes. Next I would hear a click or pop, then hum. Everything worked; it just hummed. I had recapped the unit. However, I rechecked every cap anyway with an ESR tester (all good). I started thinking about H-K (heater-cathode) shorts. I replaced every tube in sequence (two 5y3's, followed by 12at7s and 12au7s). No change; still started humming after a 'pop'. Then I replaced my six month old 5881s with a pair of Sovtek 6L6WGCs. After two hours of continuous play, no more 'pop' and no more hum. Looks like at least one of my 5881s had an H-K short. Odd that this did not show up on my Eico 667 or Mighty Mite testers. I guess the only way to be sure is through substitution. Conclusions? Great sounding tube. Failed after 6 month's casual use. Out of warranty of course.