From your description, it -might- be a bad tube(s). But there are a few other things it might be too. Give the 'shop' a chance to figure it out. I hope you trust them, and that they at least have an idea of what they're doing with that older type amplifier. Having been through that with sweep tubes, I think your best bet would be another amplifier. There's really no benefit to them, and will never be as cheap to operate as something that doesn't use sweep tubes. Suit yourself...
- 'Doc