THD as speced by the manufacturer is not the only measure of equipment
quality. For example, a lot of the HP equipment would meet spec near
their end of life or the end of life of their tubes. while Heathkit
equipment might meet spec only if routinely maintained.

HP's audio signal generators were renouned for their relatively high
power output, on the order of 1 watt for some models.

When you look at THD specs, you have to consider the whole operational
environment, including the power level, the impedance of the rated
load, and the frequency range.