I love the little Brother machines, I haven't had much experience with the multi function brothers, but have plenty with multi function copiers.
First off, how old is the machine? If Brand new, skip to the next para.. If old, are the optics clean? And does it do fine on print? if so that elemiates the whole printing option.
When you send a fax where are you sending it? Are you sending fax to email? If it is tele line, then copy quality depends on both ends. If you are scan to email or scan to network folder you can chang scan settings, ie: resolution, color/b:w, and on some machines file type. I know I can scan a jpg at a great resolution, and print at the same resolution and it come out close to the original.
Your machine has the options to scan in tiff bmp max jpg and pdf You can also scan to email image ocr file and ftp, which gives you quite the options... You can scan up to 19200x19200dpi and optical scan resolution at 600x2400 dpi, which should give you clear image, that is very ledgible. all these options are available via the user interface or the computer gui(some options may not be available on the computer gui)
You might place the original on the glass, (vs running through the doc feed.). if the quality changes but not much, it might be time to have the optics cleaned, and most copier repair places will clean for 50-100 bucks. I know it is only a 300$ machine, but with the right maint. it will last for years and cost you minimal to upkeep.
please post back with any updates. the machine will never win, never have, if I can't figure it out, it will be in pieces to figure it out, and then in the garbage as a prize winner!!