Nice signal input caps are a simple and affordable device to improve. Products such as Auricap, Sonicap, Jensen and Mundorf usually (always in my experiences) result in noticeable improvements. Mundorf silver in oil are my current favs for the money. Also, I heard nice things about Dynamicaps, although I haven't tried those yet. And then there are the Sonicap Platinums for more $$$. There are lots of choices and price ranges.
Spend some cash on tubes as well. Good tubes can make a big difference in performance.
Have fun - modest, well designed tube amps can be sooo good to you.