Is it worth it? Only you can be the judge of that. EL34 are very musical, and for the type of music you listen to they match up. Mullard and TAD are not bad options. A lot of people like the gold lion KT77 (EL34) as well.
It won’t be a significant change but it is noticeable. The stock tubes aren’t great, especially the preamp tubes. I’d replace those first and then power tubes.
The Psvane CV181 or the RCA red plate were good IIRC. Check “A British audiophile” on you tube. He does a good breakdown on the Willsenton and upgrade path.
You can tune the amp pretty warm and smooth. Should be no harshness.