When you write "noise to ground ratio" I assume you mean signal-to-noise ratio? Anyway, as another poster mentioned, there are parts missing from the pcb in the picture. From the picture you can spot three vacant surface mount components. There may be more not shown in the picture. Also, I don't know if they were not placed in the original manufactured unit (due to design changes, different models using same pcb with different features, etc.) or were removed after sale by the owner. Some appear to have some slight dis-colorization possibly from solder flux residue which may indicate the later case. If so, it is possible that a previous owner made some modification that involved removing some components from the discriminator circuit area and then lost the original surface mount components. Then, when deciding to sell the unit that owner replaced what was removed with equivalent through-hole parts in lieu of the original surface mount parts. I can't tell completely from the pictures if this is the case and don't have an original schematic but it might account for the "mod".
If that is the case then you would notice better performance from the "mod" being in place because it's not really a "mod" per-se so much as a replacement of original parts with same value but different form factor parts.
-Mike