How and why 3 switchable temperatures can eliminate/minimize the flushing?
Isn't the Profitec 400 an HX as the other mentioned machines?
Anyway i never used one of the machines, just curious to know.

After reading the Pro 400 manual, it seems to me that the Pro 400 is nowhere near as user-friendly as the Mara X is for temperature control. The Pro 400 switch just changes the boiler temperature between roughly 120°C, 124°C, and 128°C. That doesn't control the brew or group temperature as the Mara X effectively does. The Pro 400 manual even states

The PRO 400 is a two-circuit or heat exchanger espresso machine. In a heat exchanger, the brewing water is heated indirectly by the boiler. After heat-up phase and a short flush*, the specified boiler temperatures of the PRO 400 are approx. 90°C for switch position I, approx. 94°C for switch position 0 and approx. 98°C for switch position II. The measured temperatures may vary slightly, as they depend, for example, on the ambient temperature or the temperature in the water tank.

* During a flush, the portafilter is removed from the brew group and a so-called empty draw is made to free the shower screen from adhering coffee particles and to remove excessively hot brewing water (this can occur in a heat exchanger machine due to its design) from the heat exchanger. In our measurements, the duration of the flush was two seconds in each case, to ensure comparability between the different brews. It is advisable to adjust the flush duration based on what is seen and heard such as bubbling or hissing. If the flush is made longer or shorter, this changes the brewing temperature.

Switch or not, virtually every HX machine generally available to consumers requires a flushing regimen. Those that claim a walk-up first shot (with the notable exception of the Mara X) often have some serious usability problems with subsequent shots.

Profitec are chasing their tail when they design HX machines. Their Pro500 performance differs wildly between identical machines because the thermosiphon with an aggressive restrictor is a finicky system; affected by the E61 thermal state, geometry of the fluid circuit, ambient temperature and how much water you've just drawn. I think you almost need to ream the restrictor to a custom setting for every machine, if you want to minimize flushing. I've had mine for 3 years and I can't make back to back shots without a significant brew water drop. The problem with HX machines is that there are no temp monitoring probes so you have no idea what the brew water temp really is, without using sometype of homemade or professional SCACE. If you do use one, you'll be amazed as how yours performs.

There is no control system in the Pro 400 so there is no way they can guarantee any reliable brew water. As mentioned you generally need to flush an HX to cool it down just before your brew. If you have the thermosiphon running so slow that you don't need to flush, then you have a long wait before the temp rebounds and you can pull the next shot.

I'd never recommend an HX for home use. They only work well in Cafe's where they are pulling every few minutes and that cools the water.

The MaraX *IS* the only attempt I know that actually does have a control system to manage brew water.. and I think from what I read that it does a much better job of managing the brew water temp.. I think it takes some getting used to if you are steaming milk and brewing espresso as you flip modes.. but I have never used one to know those details.

It is interesting that Profitec have dropped the PID on the HX after flogging it as producing "ultra stable brew water" (words from WLL on the Pro500PID). Pure marketing hype. They also don't tell you that a group temp probe is necessary to tell when your machine has rebounded and you can pull the next shot.(if you have an older style HX that has no restrictor or not much restriction, they reheat very fast so a probe is not necessary, but you flush more often and longer)