Its funny, I've been working with the Tektronix AM503 / A6302 setup some years ago, and my experience is that they are quite accurate.
A well maintained and calibrated AM503/A6302 will easily give you precision within the limits of your scopes precision and 1-2% on your HP34401 - within the ACV limits of the 34401, crest and frequency.
We used it for work on SMPS, invaluable !, from a few kHz/100mA to 20Amps at 500kHz with risetimes that indicated well over 15MHz (fundamental) - all within 1-2%.
Remember to terminate it with 50ohms, remember to degauss, don't through it on the floor, don't use it as a hammer, don't chew on the cable, don't use it as a wrench - do I need to say more?.
Seeing how many questionable (spare)parts available on eBay inspired me to the list above
Just handle it like you would your crystal champagne glass, those you only use for special occasions - you don't through your HP3458A on the floor a few times a day either do you - and would you trust it with more than a handfull of digit if you did ?