Check out this Slim Devices Transporter mod from ModWright. The tubes make it look great, I wonder how it sounds. This is a $2,000 mod which makes the Transporter total cost $4,000.
The Transporter is re-designed from the DAC output, with ModWright's new Class A tube analog stage, including only a single capacitor in the signal path - MWI metallized oil-impregnated poly cap for output coupling. Power supply is tube-rectified, with two 6CG7's for output stage gain/buffering. 6N1P, 6922, 7308 and 6DJ8 tubes are all acceptable substitutes for the 6CG7.
Volume control and all digital inputs/outputs, wireless, ethernet, SP/DIF and toslink connections are kept intact. None of the Transporters firmware capabilities are limited or excluded and the unit automatically updates its own firmware when connected to the internet.
More information available from the ModWright site.