Elsewhere on this forum, I have seen it suggested that setting one's subwoofer's crossover frequency to 80 Hz is a good starting point, and perhaps ideal, in the typical 2.1 setup using the RTi4 as mains.
However, my setup is a little different than most, I believe: I use a pair of RTi4's in conjunction with a PSB Image Subsonic 6 in a straightforward 2.1 configuration, but what sets me apart is that my receiver has the pre out/main in thing that I use as follows: I run the preamp out to the sub using the line level outputs/inputs, and then the line level output on the sub back to the main in on the amplifier (sort of like having a separate preamp and amplifier, but in a single package). FWIW, the receiver is a Yamaha RX-V2090.
The adjustable crossover on the sub acts as a high pass filter on the line level signal passed back to the receiver's amplifier, so what ends up being amplified and sent to the RTi4's are only those frequencies above the crossover point I select.
Based on my examination of John Atkinson's yeoman's work measuring the RTi4, I understand why the 80 Hz value is proposed by the cognoscenti as appropriate where the RTi4's are run full range (and, thus, there is some overlap between the sub and the mains [viz., the RTi4's]), but I can't but wonder if there might be advantages (or disadvantages) in choosing a different crossover frequency where, as in my case, there is no such overlap.
The PSB Image Subsonic 6 is actually a pretty decent bit of kit (flat to 25 Hz, -3 db at 22 Hz, and still producing sound when my ears lose their ability to parse those tones [I was surprised to discover that I cannot hear 20 Hz and below]). It was lauded as being both powerful and musical, which I personally can attest to, and so I am quite receptive to the idea of turning over to it some of the work that might ordinarily be done by one's mains by setting the crossover higher than the norm.
The most comprehensive study I have seen on the subject of a subwoofer's frequency response and the general ability to localize a sub was that no one was able to do so when the sub was set at 118 Hz (or lower). Surprisingly, most people are apparently unable to localize higher, even much higher, frequencies. This is actually sort of consistent with my own experience and empirical research on the subject. Sort of, that is. I have no problem pointing blindfolded at the subwoofer, no matter where it is located or what is playing, even if I can't (and I really can not) really hear where the lower frequency sounds are coming from. (Does that even make any sense?) It seems that although I am usually conscious of a sub's presence and location, the sound(s) produced by the sub don't necessarily seem to be coming from the sub.
Anyway, because of all this, I could see rationales for setting the crossover frequency anywhere from 70 Hz or thereabouts to maybe even as high as 130 Hz or thereabouts. Frankly, I simply don't have the knowledge, confidence, or ability to do this unaided, and wonder if perhaps there is someone on this board who has gone down this path before? Obviously, it doesn't have to be the RTi4's, and I would think that someone experienced in biamplification and active crossovers has probably discovered some things that might assist me.
Thanking you in advance, I am your most humble servant.