Hi Carl.
You could put a softener on but the RO-Man type units are pretty good at dealing with high TDS in.
What is your tap tds and what TDS are you getting out before the DI?
I know the type of softening set up you are talking about and they do work but you will be doing a lot of backwashing of the softening resin with salt crystals.
How many litres a do you do you use? Also what is your water pressure like?
Sorry for all the questions. I need answers to give you a well informed reply.
Andrew