The K2000 digits are physically much thinner than the 34401A. I have a K2010 and have used a 34401A at work/school. I actually really like the fine digits of the Keithley, but ultimately, doesn't really matter. I think the fine digits of the Keithley will be harder to see with age, compared to the thicker HP digits. Both are constructed well. If they are at least 15 years old, I'd recommend re-capping them as mentioned above. Just beware of getting oils and dirt on the PCB as that can throw off some of the sensitive circuitry. You should clean flux residue off the board, but still try to clean as little as possible, if that makes sense.