ARC Ref Anniversary preamp and Reference 10 come with a pair of Sovtek 6550 tubes in the PS. Did anyone experiment with these tubes trying to use alternatives such as KT88, KT90, KT120 or even KT150? Ever since I bought my Ref Ann I never bothered trying anything different other than the Sovtek 6550. Does anyone want to share their experiences? Did anyone switch the Sovtek 6550 for a better alternative and stuck to it?