Nice Transparent Tube
I bought these to use in ARC Ref 300amps but I ended up having return the amps. I used them currently in a pair of McIntosh MC2301 monoblocks over the stock KT88s. I ended up preferring them as they were essentially more transparent and I liked the tighter bass. The stock KT88s had similar tonality and little more bottom end (but not as tight) and little bigger picture but were "darker", I ultimately preferred the reissue Genalex Gold Lions (Soft) which had the best of both worlds, bigger sound-stage, transparent and nice deep and tight bass. I think the Tungsol 6550s will be great in amps designed for these tubes.