Cathode bypass capacitors are not absolutely necessary in a circuit. However, their presence or absence in an amplifier affects three things: gain, frequency response, and hum.

If you feel that your amp is lacking in any of these aspects, determining whether the cathodes are bypassed is a good place to start.

We have a few Wurlitzer 200 circuit boards that we use for parts. Most of repairing vintage equipment is maintaining a large hoard. Just because a component is obsolete doesn't mean it's useless: a lot of old transistors and diodes are both absolutely necessary and hard to source. We do our best to keep a surplus when possible.