subwoofer setup problem

The REL rolloff is at 12 dB/ octave. This matches best with sealed speakers which also roll off at 12 dB/oct. Vented speakers such as the Thiel 2 2 roll off at 24 dB/oct. But I seriously doubt that they are flat to 30 Hz, most likely they start falling at around 40 Hz, in which case the ACI will blend ok. Perhaps they are set up in your room to have a peak at 30 Hz, which is good when they are running alone but a disadvantage when trying to integrate with a sub. Having experience with various subs, I would strongly disagree with the above statement that there should be an octave of overlap-- this will give an absolutely horrible 60Hz hump that might be good for rap, but not much else. If anything, the REL should have an "underlap", that is, the rolloff point on the REL should be set well below the -3dB point of the mains. In my experience, that is how they work the best. So in answer to your questions, yes, a REL will integrate much better than the ACI. A separate active xover is probably not the best answer; the good ones are expensive and if you're going to spend that kind of money, far better just to buy a bigger REL. The passive crossovers will perform better than you expect, but make sure you understand the math behind them so you can choose the proper values for your particular amp.

For Leoturetsky: low-freq rolloff is a function of the physics of sealed vs. vented boxes, and it cannot be adjusted by the designer, it just is. Sealed boxes roll off at 12 dB/oct and vented boxes at 24 dB/oct. There are minor variations in the response right around the "knee" of the curve, depending on the "Q" of sealed boxes and the particular alignment of a vented box, but they all go to the same slope below that. Listening rooms add a boost of 6 dB/oct beginning when the wavelength equals the largest room dimension, and often this coincides approximately with the rolloff of the speakers. This will modify the actual in-room rolloff of a sealed box to 6 dB/oct and of a vented box to 18 dB/oct. This is why sealed boxes have a noticeably slower rolloff in the deep bass than vented ones.

Sean: TL's unfortunately haven't been mathematically modeled the same way closed and vented boxes have, so their rolloff is somewhat open to debate. I haven't seen a source I trust enough to say for sure. One author stated 18 dB/octave but I believe this was based on empirical data, not theory. In my mind, I would think that based on how TL's work, they will be lot closer to a vented rolloff than a closed box. The difference, of course, is that they are VASTLY better damped when done properly. TL's are still an untapped gold mine, IMO. They have so much potential and so little interest. And not even the so-called experts seem to be able to give good answers to even the most basic questions about them; there is an astonishing amount of disagreement and error in the literature about the most basic of concepts and equations. Someday....

Sean, I agree, a sealed box of Q 0.5 is very hard to beat, but it's also extremely large if it's going to have good extension. It also has to be shaped and stuffed properly to absorb all the midbass and midrange output off the back of the cone, something that you get "for free" with a TL. Vented enclosures are just plain bad: poor damping and low output in the deep bass, and almost no absorptive material inside, which means the sound bounces around and comes back out through the cone and the port. All for a little extra efficiency? Yuck. I'm with you, for main speakers, give me a sealed box or a TL anyday. For subs, I'll make one exception: RELs with ARM loading, a "damped" form of reflex loading, where the frequency range is so low that it avoids these problems.