The drivers are wired in parallel but since their acoustical fields don't overlap (except low in frequency below the baffle step frequency), you get no acoustical gain at say 1000 Hz where sensitivity is normally specified. You do get the benefit of needing no baffle step compensation which can range up to 6 dB with conventional speakers. However, you do get an impedance sensitivity improvement of 3 dB (going from 8 to 4 ohms). As the basic CSS 4.5" driver is rated at 86 dB, you could theorize that these speakers are 89 dB sensitivity.

I have run these speakers with a small 30 watts SS amp with very good results (the volume control was less than half way). I suspect that you could get by with less wattage than even 30 watts.

On the upper limit of wattage these speakers do seem to go flat as you jack-up their input because thermal compression starts to occur. I suspect that your best listening levels would be under 60 watts input which should produce SPLs just over 100 dB. This level is plenty loud for a small to mid sized room but in a larger space you may wish for more loudness.

Jim