I recently purchased a set of these for my Marshall DSL40C and just had them installed. These tubes along with Tungsol 12AX7 (V1) Preferred series 7025 (V1,V2) and a Sovtek 12AX7LPS (V4) made a huge difference. I had run the pre-amp tubes separately along with the stock power tubes first and they made a big difference alone, but when adding the Preferred EL34's it took out almost all the hiss on the lead 2 channel. The amp even sounds louder and fuller now than it did before. I would highly recommend these tubes.