Testing a I/V stage using function generator
This is a pretty bad idea. How bad, depends on the DAC itself, and how its current output linearity varies with output voltage. Really, the I-V stage is the small resistor itself. Too large, and the DAC output voltage is large enough to affect linearity. Too small, and the resulting voltage will be swamped by noise.

As the DAC is the main unknown here, testing the I-V stage with a function generator won't tell you very much useful in terms of audio performance.

Better would be to use a current-input amplifier (like the traditional opamp configuration) but I gather you don,t really want to use an opamp in this project.

Fortunately there is an alternative : the common-base or common-gate amplifier has a very low input impedance (that is, the input voltage is essentially independent of input current) and high voltage gain : the ADC output current is developed in full across the anode resistor. So use that - in its vacuum tube form, it's called "grounded grid".

Easiest way would be to set up the vacuum tube stage as a normal self-biased stage (cathode resistor to ground) but with the grid literally grounded (hence the name) and AC couple the DAC output into the cathode.

(There are other tricks you could play if you can arrange the right DC conditions, e.g. if the DAC output was referenced to 2.5V and you needed a grid voltage of -2.5V wrt cathode, you could get things to work quite nicely. But I'd start with AC coupling into the cathode as a proof of concept.