Hi Mike,i use the merlin my tap tds is 315 and it brings it down to 28 thats with normal tap pressure i have thought about a booster pump to bring the tds down even more,the reason i went for the merlin is how quick it produces water for the price,there are some who have the merlin who can get a real low tds reading,
and as for all ro`s having di filters they are added as it depends on the need and what size resin vessel is required,Gaz