Do not get confused by the strange dimensions of the BSB jpg's: At 75 dpi the page dimensions are hugely inflated, the 2500x2000 pixel scans correspond to 200 dpi at the original page dimensions. These are not great, but can still be used.
Your file conversion seems to have been done by direct conversion to b/w at an identical resolution, followed by some dust filter. This results in the loss of many grey and thin lines.
Somewhere on imslp I remember simple instructions for conversion, but cannot find them now. Here are the essentials:
1) Color scans contain (obviously) more information than b/w monochrome for the same resolution. This extra information can be used if the color scan is converted to a monochrome of 2-3 times resolution. In practice, this is done by first increasing resolution of the color scan from 200 dpi to 600 dpi, and then converting to monochrome.
2) Conversion to monochrome is rarely of acceptable quality, if you use the 50% or the dither option on the mode menu. But photo-editing software also has an adjustable b/w threshold among the "settings": With a setting of "200" (instead of default "120") these Raff scans will be very readable.
3) Finally cleaning with a filter (at 2 points for the 600 dpi file) will also smooth some of the artifacts of the original 200 dpi scan.
4) Now you can convert to b/w bitmap mode using the 50% setting.
The entire process can also be automated for multiple pages, which have then to be assembled into a multipage pdf.
I will try and upload a better version of the Raff in a few day as demonstration.
I use an older Photoshop version, so I cannot comment on how to do these steps with Gimp or other freeware.
As far as I am aware, multipage color pdf's have to be split into individual pages, before they can be processed, and then reassembled.