Tim I have the scheppach 2500 older version & like you I get loads of dust under the machine. I use the Record RSDE2 connected underneath, when I split the extraction with the top guard it was absolutely useless, so I now use an Earlex WD 1000 on the top guard & although it is not that powerful it does quite a good job.
But I still cant get rid of the dust under the machine, I wonder sometimes if the record works, but it fills up pretty quickly so it must.