I tend to agree with the folks who have posted here that the 250Ti does not really need a sub. However, I have a pair of Kinergetics SW-800 subs (five 10 inch woofers per 5 foot high column) in my listening room that I used with my Martin Logan CLS ESLs. The ESLs are boxed-up in my basement but the subs remain in the room— they are too damn big to move anywhere and I’m not ready to give up on them!
So, their presence in my room inevitably fueled speculation— I wonder what the subs might sound like with the 250Tis? Initially I did not bother because the JBLs get down into the mid-to-low 30 Hz region by themselves— who needs lower than that? Finally, curiosity got the better of me and I decided to connect up the SW-800s to see how they might sound with the JBLs.
I used, as a rough guide, the basic approach documented by JBL in their Low Frequency Systems Instruction Manual for the B380/B460/BX-63 sub/crossover combination. That is, first order high pass and third order low pass for the mains and subs respectively. Subs front plane positioned within 3 feet of the front plane of the 250Tis. I used a sub-to-mains crossover of 60 Hz rather than the 63 Hz used in the BX-63 since I have a couple of electronic crossovers that can be set to 60 Hz. I have 2 subs instead of a single 'monoized' one but I think the guidelines in the JBL literature make sense to start with.
Trying the Kinergetics electronic crossover first, I noticed a large rise below 40 Hz when testing the 250tis and SW-800s using a 1/3 octave analyzer (Phonic PAA2) and pink noise. Some of the rise is due to the room (13 x 22 x 9) but I think the Kinergetics crossover is adding some EQ. I suspect Kinergetics assumed the SW-800 subs would be placed next to the CLS, well out into the room, and away from any room boundaries, hence the EQ. However, I can only place the SW-800s behind my 250Tis and up against the wall to the rear of the JBLs so a “room gain” rise in the deep bass is not a big surprise.
I substituted my Bryston 10B sub crossover (set up with 60Hz crossover frequency, first order high pass and third order low pass) and the low frequency rise mostly went away. What remains is an essentially flat response that extends all the way down to 20 Hz and which is pretty flat around the crossover of 60 Hz. An extra benefit appears to be that a shallow dip in the response of the 250Tis by themselves in the 50-100 Hz region appears to be lessened when using the subs with this setup.
BTW, using a third order high pass resulted in a markedly less flat response around the crossover region with this set-up and no improvements were noted when switching polarity of the subs relative to the 250Tis. I have not tried crossover frequencies other than 60 Hz (yet).
Of course, 1/3 octave spectral analysis using pink noise is not a high resolution measurement for the low frequencies but it does give a rough indication of performance. I am going to try some sinewave sweeps from about 200Hz down when I get around to it.
Overall, a pretty good result and I think I will keep the subs in-line for a while. In musical terms, though, adding a small amount below 30-35 Hz does not really add all that much. But there is some nice support for really low pipe organ pedals and similar kinds of music that becomes apparent when using the subs, along with a bit more oomph in the 50-100Hz region that helps some rock and jazz music. So far, I don’t hear any added nasties like bass boom or response suckouts caused by the subs.
One minor downside—I had to resort to using a couple of older Mac amps because I could not eliminate the hum and buzz I got when using my Parasound HCA-3500 and Hafler 9505 amps, despite the usual cheater plug/plug orientation games. Not a big deal.