In cases like this I think it's best to infuse with additives or add another fruit and let it take over. If there is a severe lack of flavor, sugar won't bring it out. Adding concentrate (as beernip mentioned) will help and 100L is a too much to just throw out.
"Flat" hard ciders are normally caused by the apple variety choice. Higher tannin content will provide more body and tart apples will give it a more robust presence.
If you continue to see the same problems in your hard ciders and don't have access to other apples, I would suggest boiling your cider. I would then recommend buying grape tannin and pectic enzyme (both are fairly cheap). This will raise the gravity of your cider by removing water (pectic enzyme will help in clearing after boiling), and the grape tannin will...well it'll add more tannin.