Replacing Sovtek 6922 for NOS

I have the Logos, the newest hybrid amplifier from Pathos Acoustics. The original tubes are Sovtek 6922. Any good? I suppose I could get a much better overall sound if I upgrade to NOS tubes within the ECC88/6DJ8 family. Amperex (BBoy) and Mullard seem to be very good. But I really would like to know which are your preferences. Thanks. tgm