Both are great. The Tung-Sols just sound and feel a little better to me. I first put in some Sovtek 5881s I pulled out of my SLO to see how the MOD 50 liked that tube type. I really liked it and then heard alot about the Tung-Sol reissues so ordered those. They're pretty similar, but the Tung-Sol sound a little more rich to me and have a little more sag, but this is VERY slight. They seem a little more "EL-34" to me than the Sovteks, but they are still quite stiff, clear, and punchy. You cant go wrong either way.
