H Jerome! The 15" should have a lot better bass. I think the free air resonance on a 12" is 38 Hz vs. 25Hz with the 15". That speaker you show has a volume of maybe 250-275L so that is perfect to get the speaker closer to 35Hz. That would be ideal for me. I don't need a speaker to go to 20 Hz... sure it's nice but not worth losing everything else that a low excursion Alnico paper woofer can do. Field coil versions should be amazing too but I've never done a close comparison and I'm okay with the original Alnico V Altec design.
I'd just want to clarify what "original" Altec means. For the record their primary market was selling separate drivers and crossovers, as well as sectoral horns for commercial, industrial, and custom "built-in" residential applications. 604's were typically sold
al la carte. So don't read to much into the cabinet or anything else - their drivers and horns are the "magic". And IMO their later residential speaker systems were compromises of volume (size) and some other factors to meet space, aesthetic, and price constraints.
Every speaker is going to be a set of compromises and optimizations, so you just have to go with what matches your system and tastes the best. I'd love to build 250-300L cabinets as one of the last items with my speakers, but honestly I'd like build 604's at some point - the only other speaker on my radar right now. About those Shindo crossovers... not sure if it's exactly the same but this is a similar approach to what Mast Mutter sells. But both are a different approach to the original Altec schematic. Even Ken Shindo made modified Flamencos with a new cabinet and crossover. As long as one sticks to the Altec build principles in the speaker document it is Altec. =)
Thanks for that pic Jerome... I <3 604's!!!
Shindo modified Flamenco