Everyone is of course entitled to their own opinion on the question of accepting the digitization of the entire signal when moving from the LFT-8b to the -8c, rather than just the frequencies sent to the woofers (low-passed at 180Hz with 1st-order filters). I myself am not willing to make that sacrifice. And beside which, there is a better alternative: the Rythmik Audio/GR Research OB/Dipole Servo-Feedback Subwoofer.
Using dipole woofers with dipole speakers is of course a good idea (Wendell Diller of Magnepan has for years stated that non-dipole woofers "Do not work" with dipole speakers, period. You are free to disagree with him).
The LFT-8c adds a 6.5" rear firing woofer to the front firing 8" of the -8b, still in a sealed enclosure (so not an open baffle design). The Rythmik/GRR sub is comprised of a pair of 12" woofers mounted in an open baffle frame, and powered with a Servo-Feedback amplifier. To add the OB/Dipole woofer to the LFT-8b, just leave the 8b’s stock woofer disconnected (it has its own binding post), and use the OB/Dipole in its place. The sub amplifier includes all the x/o controls (non-digital) you need to use it with any speaker you want. Unlike "normal" subs, the OB/Dipole plays up to 300Hz, well above the 8b’s x/o frequency of 180Hz. And with no digitization of the signal, at any frequency.
In my opinion, adding the Rythmik/GRR OB/Dipole Sub to the LFT-8b is a better choice that replacing the -8b’s woofer with the woofer system of the -8c. You are free to disagree
By the way, the LFT-8b, while tall (5’), is otherwise not large: only 13" wide and deep. The Sound Anchor stands are in most users opinion mandatory, which adds a few inches to the front of the speaker. ET sells the SA stand, and ships them along with the speakers. And of course all dipoles need to be at least 3’ from the wall behind them anyway (5’ is better).
As for power, though very low in sensitivity (83-84dB), the LFT-8b does not require the kind of amplification Maggies do. The -8b is a very even 8 ohm load, the Planar-Magnetic panels themselves 11 ohms. Use the OB/Dipole Sub in place of the stock woofer, and you are free to use any moderately powered tube amp you want. The Music Reference RM-9 and -200 work splendidly.
I miss being able to listen to music through my Magneplanar Tympani T-IVa (they’re far too big for my current music room (only 13’ deep and 14’/4" wide), but the ET LFT-8b’s midrange is actually superior to that of the T-IVa. And the Rythmik/GRR OB/Dipole Sub is the closest I’ve heard to providing the bass quality and character of the amazing twin Tympani bass panels, each panel 16" wide and 6’ tall. Now THAT’S a big woofer!
As for the problem of comb filtering issues between the midrange driver and the tweeter (the wavelength at 10kHz is VERY short), the speaker may be rotated to minimize any phase cancellation between the two drivers you are experiencing. I wonder why Thigpen chose such a high x/o frequency?.
