IMHO, the stock Sovteks with the SF Line series leave much to be desired - a very uninspiring midrange. Kevin Deal offers a full retube package that involves replacing only the back two tubes with first line NOS types, and the rest with NOS Russian types. If you can get them, the Siemens 7308s Deal offers did the trick in my Line 1, turning it into a virtuoso. However, Kevin is big on first talking with his customers to gauge what areas they are specifically looking to improve upon - a conversation well worth the effort (considering the price of NOS 6922s these days). I was considering white label Amperexes until speaking with him - for my needs the Siemens proved a better fit. One caveat: Kevin does sometimes have restrictions on top grade 6922 sales. Obviously, these SF models are totally transformed with a good retube.