Hi, Mel. Not sure if you saw my response to your initial inquiry last month regarding finding an owners manual for those. You can find my recommendations
here.
In addition, you can Google "NHT 2.3" and find some sites with information, including
this one that has the frequency response, nominal impedance, sensitivity, and recommended power. Just search that page for "2.3". They have both the 2.3 and 2.3a listed.