Julie, the LFT driver is a push-pull magnetic planar design (magnets on both sides of the Mylar)---resulting in very low-distortion, with low-mass conductive traces vapor-deposited onto the Mylar. The 1.7i is a single-ended design (magnets on only one side of the Mylar), it’s conductive wire glued onto the Mylar. The LFT driver has a stiff cross-braced metal frame which is bolted onto the speakers’ MDF baffle, the 1.7i has it’s un-braced Mylar glued onto it’s baffle. The LFT-8b has removable front and rear grill frames, the 1.7i a non-removable grill sock (stabled onto the bottom of the MDF frame).
The LFT driver covers frequencies 180Hz to 10kHz, with NO Crossover! Vocal and instrumental timbre remains consistent over their entire frequency range. Each note on the entire piano keyboard sounds like it’s coming from the same piano, ya know? No tonal "shifts" as the pianist’s hands move down the keyboard. That midrange driver has been in production for over three decades, without a single change! Bruce Thigpen got it right. VPI’s Harry Weisfeld stated he considers the LFT-8b to have the best midrange of any loudspeaker he has ever heard.
The LFT-8b has about the same sensitivity of the 1.7i, but is an 8 ohm resistive load, better for tube amps than the 1.7i’s 4 ohms. The LFT-8b has two pair of binding posts, making bi-amping/wiring easy. The LFT midrange driver itself is a consistent 11 ohm load, even better for tube amps. 10kHz up is handled via a ribbon tweeter. Cross-overs at 180Hz and 10kHz are symmetrical 1st order filters.
The LFT-8b plays louder and lower than the 1.7i, partly because of it’s 8" sealed dynamic woofer (for 180Hz down). Unlike other planar/dynamic hybrids, the planar midrange and dynamic woofer blend seamlessly, Bruce having invested a lot of time working on the woofer. And unlike Maggies---known for sounding veiled at lower SPL levels, the LFT-8b remains transparent at lower listening levels.
The LFT-8b has an "immediate" character, making vocals and instruments sound more "there". In comparison the 1.7i sounds somewhat "whispy": less fleshed-out & full-bodied, less viscerally "present". Hi-fi descriptions being so subjective, I’ll leave it at that. Yes, the LFT-8b and the 1.7i are both magnetic-planar designs, but they sound rather different.
It’s a shame the LFT-8b is not more accessible for auditioning. There have been quite a few reviews of the speaker in the UK, every one of them a rave. Robert E. Greene (a fairly reliable hi-fi critic) reviewed it in The Absolute Sound, and came to the same decision. A main take-away in all the reviews was the low-distortion sound of the LFT-8b, how "quiet" (no noise from spurious distortion) the speaker sounds. Pure, direct, more like an ESL than a magnetic-planar. The reviews are viewable of the Eminent Technology website.