Yes, I'm the same person.
Anyhow.
This 3458A would be rescued, only if I'm really correct in my assumption, and if an appropriate solution can be found and implemented.
These obsolete EL2018 (~5 EA in the 3458A) seem to fail after many years.
Several ADC errors also have their root cause to their breakdown.
I've seen a picture of an agilent original (?) replacement of these comparators on the ADC board, by an SMD two fold fast comparator, (piggy back) but I cannot remember the correct site, nor supplier, nor type designator, I'm so sorry.
Maybe anybody else can help.
Otherwise, give the Chinese sellers a chance.
Frank