Originally Posted by S_rangeBrew
I respectfuly disagree with my fellow IB user. I have 4 ib3s in a 1800 cf room. They are not even close to bottoming. Also i use a crown xls 5000 for 500 watts per driver. No problems. Plenty of 5 hz sound.
Get what you can and leave room for expansion.
When I bought mine, I free air tested them, individually, in pairs, and in a group of four. With a Behringer EP4000, as I raised the drive level, I could ease them into bottoming prior to full power in both the 4ohm config (one amp channel per 4ohm driver), and the bridged 4ohm config (two sets of series connected drivers, paralelled across the amp. In both scenarios, I could tap the VCs on the back plate.
The EP4K puts out 650w per channel into 4ohms, and ~1600w-2000w into 4ohms bridged.
I tested them once fully installed too, in various configs, even with a second EP4K,..to allow one driver per channel operation. The only config I can operate in without worrying about bottoming, is series connected 8ohm loads,...which is 450watts per channel, 225watts per driver.
I can't explain the differences, however I believe you. Something is influencing the difference. Maybe the signal chain possesses some roll-off, or the drivers are different. A year or two ago, I inquired specifically with the designer/builder at Fi (Nick I believe), he stated there's no differences in either of the two versions of IB3-18s with regard to the excursion capability, Xmech, Xmax, clearances, etc. They re-tooled and thus, I was interested in what was going on wrt the driver.
No biggie, as it's still plenty strong