Its no secret - most people who have used them will tell you its a NILFISK GM80 - with hepa addons - VERY powerful for a tub vac, in fact just VERY powerful - also very quiet and easy to haul around - simple to maintain and they LAST FOREVER
buy one and you will become a convert - I guarantee it - only drawback I find is with the hepa filter the exhaust comes out the top and if you are carrying while doing stairs for instance it does warm your carrying hand up a tad - perfect for curtains though I would think and loads of tools
Oh and BTW they are very expensive but there are several on sale on ebay at present and they will probably go cheaply
dave