I'm looking at getting another pair of 6sn7 for my Rogue 99 super Mag pre amp in the amplifying position.  Considering Raytheon 6sn7 GT VT-231 vs Sylvania  6sn7 GT VT-231 (similar or even identical  to bad boys I believe).  There are others I would look at but they are beyond my price point. 
Anyone compared these?
my system is Squeezebox Touch, NAD CD-  99 pre amp- Rogue Stereo 100 amp- Salk Song towers + Rythmik sub. Sounds  very dynamic, airy, not too colored..  Would be nice to have a slightly lush or  warmer pair of 6sn7's for rolling. Also must be very quiet as the 99 is sensitive to noise- although I can always return if they are not. 
Currently I have 2 short RCA's combined with 2 Sylvania tall 6sn7 GTA's.   Is there discernable difference between Sylvania GTA's and the VT- 231's?  (the GTA's are sweet and very low noise)
many thanks
Tom