“A downward-firing 12in carbon ‘SuperProgressive’ passive radiator again uses a new design intended to produce extremely long travel while retaining the variable stiffness of its suspension. The thinking here is that it enables the S/812 to act like a sealed-box compact 12in design at low volume, but still deliver bass down to 19Hz when pushed. At the rear are the inputs, including high-level Neutrik Speakon, low-level stereo phono, and LFE (phono and balanced XLR) connections. There are also high-level Neutrik Speakon and LFE (phono and XLR) outputs.
Otherwise the back plate is surprisingly sparse, with just two switches (On/Standby, and 180-degree phase), and knobs for high/low/LFE levels and crossover. The Serie S subs are also compatible with REL’s new wireless transmission system (AirShip).
These are the first mid-sized RELs designed to be stackable, thanks to special fixtures and included metal plates. You can safely stack and lock S/812s up to three units high. That sounds bonkers but there is logic in REL’s madness. In the real world, bass has width, depth and height; by stacking these subs you can create bigger bass presence from a smaller footprint.”