Thanks for the kind words! There were generally three versions of the 400C, as identified whether you have a single crossover setting, dual crossover settings (very rare), or no crossover settings, with these last units to my knowledge always employing the dual concentric tone controls (earlier versions employ single knob ganged controls). The point is, determining which of these units you have will also help to determine how much hum you have. The last generation had the most (as well as some other issues as well), while the earliest models had the least. Properly modified however, any version of the 400C will emit nothing but a soft hiss.
Hum always interferes with detail presentation, most obviously of course at low levels, but in the 400C, there are other elements to effect that can affect the sound as well:
1. The earliest units had both phono inputs reflecting a cartridge load of 100K. Later units had the RIAA 1 input reflect the newer 47K standard. This can have a significant effect with some cartridges.
2. There are built in permanent LF filters -- besides the switchable LF filter -- that can hinder response as well. Now back in the day, it was hardly a concern with the changers and records of the day, and many of these units were used in console settings as well. Today however with the best mounts, tables, arms and pressings today, having full response throughout the system is important. Removing these filters is a big help towards improved definition.
3. Fisher also generally tightened up the rather typical RIAA deviation of the day (that is, as designed into the 400C) in later units (after the 400C), but even then it was still loose by the best standards of today. There is a thread you can reference on the Fisher Forum here where I developed a highly accurate network for the Fisher preamp (same configuration, different values) that maintains a very tight compliance with the published curve with just a very small deviation (.25 db as I recall) over the full 20 Hz to 20 kHz bandwidth. This was confirmed BOTH by measurement, and by expert modeling work. That thread might be of interest to you as well, which you can find here:
http://www.audiokarma.org/forums/index.php?threads/the-fisher-phono-preamp-and-riaa.600690/
I hope this helps!
Dave