Just a follow up. I picked up my second 606 today this is the one with the 604C in it. The first cabinet I bought has a 601 in it that I have to replace with another 604c. I played the 604 today and had the opportunity to A/B it with one of my two Altec Valencia's that I added GPA HF drivers to and new crossovers that I built using high quality components. These Valencia's are near the top of as good as a Valencia can get. When testing I had the 604c on left channel, Valencia on right channel, preamp set to mono A+B. Tube preamp, Williamson monoblocks at about 30 watts per. The 604c has a bone stock crossover and compared to the Valencia it really shined. What an amazing sound. The combination of speaker and cabinet is wonderful. The first thought that came to mind when listening was "what presence". The music just jumped out at me as if it was in the room and not in a box. I can't wait to get to work on some custom crossover's with time alignment. I'm going to be looking for another 604c to add to the second cabinet so my need to figure out the LF resonance of an E and how to modify the cabinet accordingly won't be needed.