Paul, the feet of your undercounter combo are supposed to slide into tracks that are attached to the floor.  I remember getting a friend's GE combo to spin at "full" speed by sitting on the left front corner of it to keep the speed controller attached to that foot from slowing the speed as the machine rocked on it with a less than well distributed load.

Doumatics only spun at around 250 in the initial minute of spin, then ramped up to 500 rpm. They could do that because they had a suspension system of springs and shock absorbers. Bendix engineered the Duomatic while AVCO owned the company so AVCO held the patents on a suspended mechanism for washer dryer combinations. Avco also held what is called the intellectual property rights of a washer-dryer combination so anyone seeking to manufacture one had to pay royalties to AVCO for every combination produced.  Making combos in the United States was living under a deal with the devil that caused most companies to lose money on them. They had to engineer a hobbled machine and then pay money to AVCO before they ever sold them. Your mother's Westinghouse combo spun at around 200 rpm. At 350 rpm it could have danced out of the house or at least to the end of its hoses or power cable. When WP redesigned the combos they made for their own label and for Sears, they went to a complicated water balancing system with feedback to air-driven clutches in the transmission to correlate balance with speed, not allowing the machine to go into high speed spin (400 rpm) until the cylinder was in balance.  This avoided running afoul of the AVCO patents while providing water extraction that was almost on par with the Duomatic.

The smaller drums of the Mieles with sophisticated software do tend to balance loads easier than machines with larger drums because there is less surface over which to plaster the load. That is why the longer drum for the 6.5 kg machine makes more sense than a wider drum to say nothing of keeping it in the same 24" cabinet.

The earliest of the 29" combos did spin at 500, but the machine did not have a lint filter for the dryer that could be cleaned; it was one of those separation things that then flushed everything down the drain and when the machines were rebuilt to get rid of that and have a transmission that shifted action between the pump and the blower, the new transmission only spun at 400rpm.