After looking at the Willsenton web site I see that they use generic EL34 tubes stamped with their Willsenton name. (I'm sure Willsenton is not manufacturing tubes.) At that price point they are likely not higher quality tubes. Even expensive tube amp manufacturers often use "budget" tubes expecting the buyer will roll them to higher quality tubes.
So if you haven't already done so I would recommend two matched pairs of Tung Sol EL34's and also upgrade the 12AX7 and 12AU7 vacuum tubes.
So if you haven't already done so I would recommend two matched pairs of Tung Sol EL34's and also upgrade the 12AX7 and 12AU7 vacuum tubes.