I have tried a ton of different tubes in the VK5i. I found no problems with the 6DJ8, but mostly I used 6922's I actually prefered Mullard over Amperex. For what it's worth to my ears there wasn't much difference between the high buck Amprex's and the less expensive Bugle Boys.
I found out something very interesting though. You can achieve excellent results by only replacing the 2 very outside tubes ( towards the front ). I found this to take that bit of glare of the mids, and highs that the Sovteks had. And also it was much cheaper to only replace 2 vs 6! In replacimg only the 2, I found Telefunken to be the best choice.
Another thing you can try is to replace the two 5881's ( you can use 6L6 tubes here ) In this case I liked Sylvania 6L6GC's But there were many others that produced a pleasing tone.
Of course all of this happened in my system, and with my taste for sonics and music.
I found out something very interesting though. You can achieve excellent results by only replacing the 2 very outside tubes ( towards the front ). I found this to take that bit of glare of the mids, and highs that the Sovteks had. And also it was much cheaper to only replace 2 vs 6! In replacimg only the 2, I found Telefunken to be the best choice.
Another thing you can try is to replace the two 5881's ( you can use 6L6 tubes here ) In this case I liked Sylvania 6L6GC's But there were many others that produced a pleasing tone.
Of course all of this happened in my system, and with my taste for sonics and music.