Hi! I've replaced my old stylus on a technics sl 20 (EPS 270 ED) by a tonar 757-DE elliptical, as it was recommended. The result is disappointing. The lows seem right, but the middle and higher range is very distorted. I use a Yamaha amplifier. What could be wrong?

If you mid and hi's are distorted you are having tracking issues after replacing the stylus.
Three important thing to note is vertical tracking angle , azimuth and tracking weight.Did you struggle with the tone arm while replacing the stylus ?.You could have disturbed all these.
1) Any way check if the tone arm is parallel to the record when its playing. Look from the side of the turntable. If you see any difference put back the old stylus and check.
2) View the cartridge from the front, it should be perpendicular to the record. If you have a small mirror place it on the platter and drop the needle on it ( with the players power OFF ) .The reflection should be perfectly perpendicular to the object.
3) Try adjusting the tracking weight by turning the counter weight at the back of the arm. You might have to increase the weight to allow better tracking

You definitely want to make sure that your new cartridge is properly aligned for your turntable. This web site can help:
http://www.audiophilia.com/features/cartridge_setup.htm
If you don't feel that you are able to get it properly aligned by yourself, a local dealer should be able to help you with this for a small fee.