Pro-ject tubebox ii : muffled sound

I found a Tubebox ii at a thrift store for a price that I could not refuse.

The sound is muffled, like it is so warm it is in the next room. It still has the original tubes -- is this something that a new set of tubes will likely fix up, or do I need to take it in?

To that end, it calls for 12AX7A tubes, and I'd love any input on choosing replacements (I'm new o tubes) -- the price range and lack of adequate descriptions for tubes is befuddling, but I assume the differences are significant. I can spend anywhere $20 to $2500... hmm.. it'd be nice if the $20 pair worked..

I've been using my McIntosh MA6500's built in phono stage for about a year, and have been happy enough with that.