At the beginning of 1989, the Power Plus series was introduced. At that time, the lineup was expanded with the 3.3-liter 455 model, which had a power output of 67 hp. At the same time, the smaller model range began to offer five different color options. The hydraulics were updated: the housing of the lifting device was enlarged, allowing the auxiliary cylinders to be omitted. All models now featured a single-lever fan, and the lifting capacity of the 305 also increased to 2,500 kilograms. Like the larger models, the turbochargers were replaced with a more modern design, which reduced turbo lag.

Soon after this, the model range was expanded significantly. The smallest model became the naturally aspirated 205 with 49 horsepower. It had a slower transmission than the other models, which was also marketed in some countries for use in the larger models. At the beginning of 1990, between the 305 and 405 came the naturally aspirated 3.3-liter 355 with 60.5 horsepower and at the top of the series in the summer of 1990 the 72-horsepower 555, equipped with a ceramic clutch. It was also the only small model at that point to offer an electric lifting device. The small Valmet tractors still featured draft control by push rods, which was quite rare to combine with an electric lifting device.

Production of the 205, 305, and 405 models ended in the summer of 1991. However, at the end of 1992, the 255 was introduced with the same power as the 305. The difference from its predecessor was a 3.3-liter naturally aspirated engine and the transmission of the 55 series. The next innovations were the 65 series with a redesigned cab, which gradually replaced the 55 models from 1992 to 1994. In the last model years of the 55 series, there was as standard a choice of either a 540/750 rpm or 540/1000 rpm PTO. The 55 models came standard with two double-acting hydraulic valves, except for the 255.

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