[DaleHCook]

I am using three such attenuators, a 20 dB, another 20 dB, and a 14 dB to reduce the output signal by 54 dB.

Ron - The HP 355C (0-12dB in 1 dB steps) and 355D (0 to 120dB in 10dB steps) step attenuators are more elegant solutions. I have long owned both, largely for use with GR unit oscillators which lack step attenuators.

... do the HP generators produce a low level signal usable in vintage receivers without overloading them?

They do - around less than 0.1 microvolt into 50 ohms is not unusual.