I am familiar with Teseq ones, super easy to use so techs can do the testing. It's a very high cost of ownership with commercial mains transient generators.
A new standard rolls out, or an update to a standard and you have to send it in for hardware mods- or the unit just goes obsolete. Steady churn of these units on the surplus market.
Calibration is a constant cash drain, for test-lab requirements. New, they are the price of a car.
That's why I wanted joeq to give details about the insides of his homebrew one
If output voltage is low, make sure the unit is capable of that in the first place. It could be for older (standards).
It could need repairs, have some damage and only be making 1.6kV not 2kV, or have high output resistance. Or there is an arc in the DUT.
I'd test it with a simple resisitive load to make sure it's working properly.
Most important is safe test leads and the blast shield, on an insulated bench. You have to know what you are doing. We wouldn't want a dead Treez.