I'm using 3 HGS-18s across the front of my main theatre. I tried the 15s as well. I highly recommend the 18s over the 15s for a more literally 'stunning' result. Was using M&K 5000s and 350s. The 350s are on the sides and rear channels now and the 5000s are in the master bedroom system. Room tuning/treatment became more critical with the larger driver Velodynes than the M&K dual 12" design.
DrM
DrM