Marshall paid royalties to Groove Tubes (USA) to use the patented circuit. Eddie Van Halen and others have used this or the Groove Tubes equivalent on stage for years. According to info on the net, all the big studios in LA are using them and they are quite rare and expensive nowadays if you are able to come across one on ebay or other places. I bought them new in the early/mid 90's.
My 1987x 50w plexi MK II Marshall speaker output runs at full volume into one of these and then a "low" line level output signal is sent from the SE-100 via stereo effects to each 100w plexi Marshall. You don't need any cabinet connected to this speaker emulator even with an angry 100w Marshall running in to it at full blast. I only use them as a "dummy load" and I prefer using mics for recording. However, the SE-100 are able to simulate a mic position at 0, 30 and 60 degrees + open/closed cab. As for power attenuation (if a cab. is used) it reduces it to max -18dB.
The output impedance on the amp must be set to 8 ohm to match the SE-100's input but the choice of cab. type (and ohm) however are not relevant (as far as protection of the amp) unlike the THD Hotplate where the ohm setting must be equalent all the way trough. The Hotplates are very good though because you can attenuate the volume trough the cabinets all the way down to -oo dB if needed (no sound) which comes in handy both for recording as well as for gigs at smaller clubs etc.