I have an ARC LS15 with Solvtek tubes. It's horrible sound, the mid sometime hollow. It's missing something in the middle od soundstage. I recently replaced with two Amperex 6922 PQ and two Amerex 6dj8 buggleboy. The sound is heaven. Way way better than Jan Phillips 6922 and the stock tube Solvtek. Huge, wide soundstage. I recommend if you can find some Amperex tube somewhere in e-bay, 6dj8 is okey too. This would be the vast improvement. I paid mine for total of $45. $30 for Amperex 6922 PQ and $15 for the two buggleboy. To me the 6922 and 6dj8 sound about the same. The 6dj8 buggleboy has more bottom and punch.