Has anyone used these speakers ? In JMLab SIB production sheet it marks nominal impedance as 8 ohms. However, the information in JMLab SIB user manual says the nominal impedance is 6 ohms. Is this speaker 6 ohms or 8 ohms ? My receiver must take speakers 8 ohms and up (can't be lower - will burn receiver). Is there any technical difference between current version (black jet finish) and its previous versions of Technic (black/silver) & Classic (wallnut/silver) ? Thanks