I just acquired an HP 8640B and now I'd like to get rid of my HP 606A but I
need to go below 500 KHz.

Hank-

Recently someone mentioned that the 8640B was a UHF signal generator that had
digital dividers for lower frequencies. If that is true, you might be able to
add additional dividers to go lower in frequency. You might have to sacrifice
one of the existing bands, depending on how extensive the modification is.

73, Fred, K4DII