Laminate underlay?

We were allowed to put down laminate in our 1st floor flat with the proviso that if downstairs complain about noise, we need to take it back up again and lay carpet. So, we've found the laminate underlay with the highest DB sound muffling rating we could find, the Tredaire Technics 5 (http://www.ukflooringonline.co.uk/product/tredaire-technics-5) and are going to fit it under the Hygena laminate we bought.

We've now found though that some places list the Technics as a carpet underlay, and are worried in case it has too much give in it and will crack our laminate. I read somewhere about putting the underlay below, say, fibreboard to give a stiffer surface, but I don't know if that's a good idea? Is there a particular type of fibreboard that would be best? Floor is currently stripped back to bare floorboards in a 1890 house and you can hear a LOT from downstairs when someone walks across the room.

We have the laminate and the underlay, have yet to buy the fibreboard - it'd be brill if someone can help. We don't want to have to take up the laminate due to noise, or damage it. Cheers!