For final specs, the 29mm Satori silk-dome tweeter and 165mm Satori mid/woofers hand over at 2'500Hz with a shallow 6dB/octave phase/impedance-corrected 1st-order filter. Nominal impedance is 4Ω, min. Ω is 3.2 at an inconsequential 18'000Hz where power demands on the amplifier are marginal. Internal cabling is "recrystallized laboratory silver with anti-electrostatic dielectric". Pair matching is better than ±0.25dB. The bass reflex port is tuned to 29.9Hz. Warranty is 3 years. "In both Sigmas the tweeter is driven only through a very high-quality capacitor. The mid/woofers are driven only through a very high-quality linear inductor which becomes a tape cross-coil inductor for the 4, a solid thick-wire air-core inductor for the 2. Nevertheless, there are many other high-quality crossover parts to equalize amplitude as well as phase. Other parts equalize the impedance. That is profitable especially for users of tube amplifiers. That is also why the terminal plates on both Sigmas are so large."

To recap for emphasis, contrary to the minimum parts dogma of certain 1st-order adherents whom we might call the single cap+coil brigade, sounddeco propose that it's ultimately better to go a bit further and iron out unnecessary wrinkles in the phase, frequency and impedance plots. Not only does that benefit measured linearity, it decomplexifies the amp/speaker interface—in proper Schiit Audio talk, it actually decrappifies it—to also make for better sonic performance. As tube amp fans particularly of the zero NFB SET persuasion know well from personal experience, it's often not the power conversion efficiency which determines whether a given SET can properly drive 'x' speaker; but the hairiness of the latter's impedance and phase plots. Casual talk of easy loads refers to minimized phase angles and shallower impedance fluctuations. That's particularly relevant with ported designs of low tuning. Their typical saddle response in the impedance domain can show differences of 50 ohms in a power-hungry band. And that's so not what a SET with high output impedance wants to see.

Sigma 2 setup of Premium Sound in Gdansk as shown on the Stereo i Kolorowo Underground blog here.

Grzegorz Matusiak who oversees all of sounddeco's acoustic design work holds an Acoustics Ph.D. from the Wrocław Tech University. His research papers have been published by the Audio Engineering Society and European Acoustics Association. They include work on the application of the reactance transformation method for the design of band-pass loudspeaker systems; 4th, 6th and 8th-order active and passive symmetrical band-pass loudspeakers; fundamentals on loudspeaker radiation vs. acoustic quality plus speaker efficiency. He holds two patents, one on a bandpass speaker device, the other on a speaker dome made from bacterial cellulose. Prior design work includes a role as assistant director of development with the Polish Tonsil Co.; and ongoing work on active studio monitors for Audio Pro Solutions.

He traces back his love of the acoustics subject matter to Żyszkowski's "Fundamentals of Electroacoustics" text book which he chanced upon in high school. A later useful discovery was a master's thesis on sound radiation from the Physics Institute at Rzeszów university. In his design work, he considers all the usual measurements, radiance resistance and mathematical distortion modeling using non-linear differential equations. He believes that the diaphragm materials of drive units are very influential on the final sound; and that all available tools including critical listening are equally valid and should be exploited together. His Sigma range for sounddeco matured over two years and might have bowed at the 2013 Warsaw show had the team signed off an early results. The decision for the Sigma d'Appolito array was subservient to desired directionality which is optimized for a seated position. With the larger Sigma 4, perfect focus of the five drive units occurs at 3.5 metres. The Sigma 2 is suitable also for smaller spaces. Given currently nude ends on the Sigma 4's bowed baffle, one might suspect a Sigma 6 on the books which would add another mid/woofer to either end of the current line array.

In the left shot we see the dual rectangular ports of the Sigma 4 which Grzegorz mentioned earlier.