To turn the LFT-8b in an absolute giant-killer (actually, even in stock form it is), use a pair of GR Research/Rythmik OB/Dipole Subs in place of the LFT’s 8" woofers. Unlike "normal" subs, the OB/Dipole will play up to the 8b’s x/o frequency of 180Hz.
Until you do that, there is a remedy for the resonance in the 8" woofers output (in the 1k-1.5kHz range, way above it's x/o frequency), heard as a low-level "whistle". "Davey" has posted on The Planar Speaker Asylum a simple, cheap low-pass filter that suppresses that resonance to the point of inaudibility. It is just a single cap and resister, placed across the woofer's terminals (on the back of the woofer, not the speaker terminals).