Which Preamp Tube is most consequential: Input or Gain

I purchased one of those preamps just as Schiit Audio switched to the Freya+ and the Freya (no tube option). I started with the Tung Sol 6sn7 option instead of the Russian NOS 6sn7s. I have since switched to some G.E. 6sn7 GTB tubes from Canada. In my system they have done a better job of separating the instruments, although the Tung Sol had a bit better detail. 

I switched all four tubes though, not just two. Now I have something new to try.

Thanks @rickytickytwo . @ghasley The impact of the Tung-Sols on the right was much richer bass, on the left the bass seemed good but more in balance and everything else got nice and airy with good soundstage. So while rolling better tubes might not shift things much, stepping up from the stock 6Sn8s did. I guess I'll eventually pick up another pair and replace the remaining 2 stock tubes.Thanks for the vendor recommendations!

Place your best tubes on the right hand side forward and rear

Why would you install the new tubes in the right channel gain and cathode follower?
The new tubes go in L and R channel, either front or back.
Front would be the gain stage, unless Freya has an odd design.

Thanks, @lowrider57 , the Freya has two tubes per channel. Forward and rear are L/R channels; while the two left tubes are Input and two rights are Cathode follower. So I've been changing the two inputs or the two outputs to keep the channels the same.

The input tubes are always the place to start.