If these printers have poor contrast, usually its caused by the paper (or a dirty printhead).
The old paper has aged way beyond its intended life and as such, has become insensitive, and recently made paper is intended for different printheads (usually much hotter, especially for high speed POS reciept printers), so unfortunately its really hard to find anything that works well. If there were some adjustment to increase printhead drive (or increasing its supply voltage) it might help, but it also might burn out the head.