I've got a ton of MTS in a tank that I keep the pH from 5.6 - 6.4. I don't notice any difference in them from when the tank was 7.00 or higher.
Edit: My current TDS is 50 - 60 ppm. It used to be 100 - 110.
I might be a little late to this thread, but I have a Eheim 2227 which is identical to the Eheim 2229 but without the heater.
It's a great filter. It's been in continuous use for almost 15 years and only gets cleaned every 6 months or so whether it needs it or not.