6550 Replacement Tube for ARC Reference 6

I am reviewing info on the best tubes to replace the original ones in my ARC Ref 6. There is one 6550 tube used in this preamp, and it appears that the best replacement is a Tung Sol 6550.  However, there are different grades for this, including different grades for the NOS versions.  Does anyone know the different grades of this and the different sonic qualities each of these would have?  I did note that a couple people claimed that the cheaper reissue of this tube is less detailed, but has a somewhat richer sound.  Of course, all this is a matter of taste--any thoughts?  (I never bought into the notion that more expensive is always better.)  Also, any suggestions for alternatives to the original 6H30 tubes?  I understand that these are all made in Russia; however, are there any makes better than others for these?