Had an Eastern Electric DAC that had a tube which could be switched in and out. Even with the recommended $50 upgrade tube could hear no difference with or without the tube.
With D/A technology changing so fast I won't over invest in a DAC.
In my limited experience: NOS seriously lack dynamics, input tubes are for tube lovers and designers who prefer to design with them. The real questions for DAC shoppers are: is DSD/MQA important to you; choosing between R2R, delta-sigma, or FPGA; and if you desire eliminating a separate preamp.