I think they run about $400 (more if you want the better outboard PS), or you could simply get a DAC with a tube output stage, like an AcK DaC.
I use a MF Tri-Vista DAC, which has teensy milspec tubes in its output stage, and really good DAC. New they were $2400, but I see them all day long around $1200.
If you wanted tube sound without a tube pre-amp or amp, thems would be my choice.