They are in parallel on each side so the resulting impedance is 4 ohms. They are driven by a Soundcraft SA-1000 professional stereo 1000 watt amp that puts out 500 watts per side into 4 ohms. I do have another identical amp and for awhile drove each side by it's own amp, but I found that I didn't need the additional power. In spite of the port plugging, they are still quite efficient in the scheme of things because of the large cabinet size and 18" drivers.

The tuning has always been the same. The pictures I have posted before have been with the grilles in place.

The resulting response in-room is dead flat down to 10Hz, which is the lower limit of my RTA program (TureRTA).