a very small price to pay for the increase in quality bass, and if the sensitivity is a factor, double your power to gain your 3-4 db. in alot of cases its possible to yeild better bass with a much cheaper woofer, than using 1 more exprensive woofer, some of the best bass ive ever heard in iso was with cheaper more efficient woofers that had great attack due to low moving mass, but the woofer still has to have low extension abilities. its too bad that t-lines are so complicated to desing and build right, oh well the are advantages and disadvantages to every type of alignment, and in the end it all depends on what you are trying to achieve with your sub system, i agree with everything you said but in the end the speaker and sound we choose to accomplish will determine the best alignment.

 

like i said, it all depends on the desired result, the doubling your power to gain 3 db isnt folklore, as long as the amplifier will double its power when halfing the impedence., but unfortunately very few amp will actually double power so you would typically have to use another amplifier. what configuration did you have you rockford 15's in, compoung, push pull, vented, or sealed, have you ever used the speakers individually in sealed or vented, and if so, how did it sound compared to your iso system

 

Also it doesn't help if your subs are putting out their max SPL already. I used a clamshell isobaric setup with 2 MTX Thunder Pro 10's way back in the day. It was in a ported box designed using the loudspeaker design cookbook, in an SBB4 alignment. The box wasn't exactly half the size of a non isobaric box but it was quite a bit smaller. It was the best sounding box I had up to that point, but my next setup with 2 Soundstream SS10R's in a sealed box sounded better and got quite a bit louder.