National SW-54 Thoughts

[Notimetolooz]

OK. You are still kind of getting ahead of yourself. If you tested C10 while it is still connected then the reading will be wrong. The reading would be effected by other components. You should ask about things here first, then if necessary order parts. At this point you don't know enough to make some decisions before forging ahead.
Wow, so that Digilent Analog Discovery can do a lot of things, it isn't just a oscilloscope. In a way that makes it harder to figure it out. You can change things around a lot in the software. Since I don't know the software I don't think I can help with setting it up right.
One thing that I did notice is that the maximum input voltage is +-25V.
That voltage is fine for most modern solid state circuits but tube circuits can have hundreds of volts DC on them. Unless you use a DC blocking capacitor on the inputs you could easily blow the scope input circuits. I would use a 0.1uf, 400 to 600V film cap.
I notice that there are trigger inputs on that device. They may or may not be what you should use with the scope inputs, the trigger inputs might be digital inputs used when the unit is used as a logic analyzer.
Probably few people on this forum know how to use that unit, you might get better understanding on it somewhere else.
The frequency and input resistance are fine. It may not be very sensitive to small signal levels however, but tube circuits do have a high signal level in general.