My 2c
I've got both a D4 and a Felix. Their airflows are similar at the power head. In regards to suction the Felix is stronger the D4 had about 70" at the hose vs the 76" of the felix. there's a leak around the bag indicator that causes that
Between the two machines I think I prefer the D4. Why? It's quieter, the non stretch hose makes above the floor cleaning easier, the bags are larger, the cord winder is a nice convenience.
If you prefer a one piece machine though you may be better off with a Felix, but IMO you should get the Premium parquet tool 7200GS, remove the front strip, and use it with your D4.
I'm more likely to use the felix's floor tool than the basic floor brush that came with the D4
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