Dsper, I assume Sidsip is referring to the resonance frequency of the combined Shelter/SL-1200 arm. It's a complicated subject, probably beyond the patience of anyone to explain comprehensively in a single forum post. In any case, it would be good to educate yourself on the topic. Just Google "cartridge tonearm matching," and you will find a wealth of information, including the following:
http://www.theanalogdept.com/cartridge___arm_matching.htm
http://www.gcaudio.com/resources/howtos/tonearmcartridge.html
http://www.theanalogdept.com/cartridge___arm_matching.htm
http://www.gcaudio.com/resources/howtos/tonearmcartridge.html