I assume (believe) HighVoltage is now satisfied with the stable solution he has for all his DMM Agilent/Keysight units.
Actually, you are right.
At this time, I am completely happy with the Free Software provided by RF-Scientific and the Prologix GPIB to USB adapter.
It works so well, I can hook up many of my Agilent meters at the same time.
So far I have only played with three 34401A meters in addition to the 3458A.
But all I can say is, that this software works great!
It is highly recommended for anyone that wants to get data out of the 3458A quickly and without programming and still have control over the commands sent to the 3458A.
I will keep the 3458A running for one to two weeks now and see if it will change of stabilize the CAL:?72 10V DC constant.
It really seems, this 3458A was bought by the first owner and never used.
It seems, once in a while we can get very lucky, buying something used, even a high end instrument.