OK, some stuff just trickled back into my brain--and I found some photos that I took of it in action (it went on eBay). The manuals are not well organized for figuring this stuff out. Maybe it's not broken after all.
There are controls for 'synthesis mode' on the lower right. Only two of them work unless you have the option for enhanced spectral purity (don't remember the number). I don't know how much, if any, effect this will have on harmonics.
Do you have Option 002 (Frequency doubler)? I think the harmonics are worse with that option.
I think the harmonics will be higher at higher drive levels. Drop down 5dB at a time and see what the harmonics do.
I still haven't found the spec sheet that
TimFox mentioned, but -25dBc at 100MHz seems high, but maybe it doesn't matter for its intended uses.
EDIT: Here it is, in black and white. Guaranteed specs for harmonics are only <30dBc @ <10dBm.https://www.testequipmentdepot.com/usedequipment/pdf/8644b-8664a-8665b.pdf