Having just bought a pair of NS-15 here in Switzerland and looking around for more information on those I came across your thread and the NS-serie of Ymaha speakers and found the following link which might serve you
http://audio-heritage.jp/YAMAHA/speaker/ns-30a.html
stating for NS-30A and NS-30B
50Hz~20000Hz
200Hz、6000Hz(6、12dB/oct)
8Ω
103dB/W
30W
http://audio-heritage.jp/YAMAHA/speaker/ns-30a.html
stating for NS-30A and NS-30B
50Hz~20000Hz
200Hz、6000Hz(6、12dB/oct)
8Ω
103dB/W
30W