Marco
From your post it seems to me that you are hearing something different rather than something better. The loss of bass with your tube IMO is one of the reasons ARC goes with their provided 6550C and then for you to have to adjust speaker position suggests to me that whatever difference you heard was not related to the 6550 Tungsram but rather speaker realignment. It makes no sense to me that by changing a tube you have to change speaker position and the feet on the Ref3 for better listening pleasure. I might suggest to you to put the original 6550C back in, keep the speakers where you now have them and keep the tweak on the preamp feet and I would bet your bass is again back to where you like it and overall the sound is better. By inference therefore the speaker repositioning and tweak is what changed, nothing more.
From your post it seems to me that you are hearing something different rather than something better. The loss of bass with your tube IMO is one of the reasons ARC goes with their provided 6550C and then for you to have to adjust speaker position suggests to me that whatever difference you heard was not related to the 6550 Tungsram but rather speaker realignment. It makes no sense to me that by changing a tube you have to change speaker position and the feet on the Ref3 for better listening pleasure. I might suggest to you to put the original 6550C back in, keep the speakers where you now have them and keep the tweak on the preamp feet and I would bet your bass is again back to where you like it and overall the sound is better. By inference therefore the speaker repositioning and tweak is what changed, nothing more.