there are little to no sellers of these Passive LSR705i or LSR708i
If they are not selling or garnishing reviews because of lack of interest, then I would guess that they will be discontinued or replaced with active versions. Maybe the hype about the smaller versions has sucked up all the oxygen. The 7 series price compares with other mid level professional mixing monitors, but the reviews of 3 series say they are better than many much more expensive monitors. So why buy a 7 series? Especially since same size woofer, so no LF dynamic advantage. The bigger horn of 7 series lowers xo freq, allowing louder playing at same low distortion level, but indoor playing at 85dB for audiophile use this is not a concern.
I wonder how high off the ground would the LSR708Is need to be for optimal sound?
Wherever you get the best image illusion. I personally like the tweeters 6-12" above my ears.
Also would these only need to be used in a nearfield setting?
They will work in far field. The horn will reduce reflections to reduce room distortion especially from ceiling reflections, but also sidewall reflections if you toe them in.
I daydream about these speakers in my other system but I'd be sitting about 10ft away from them.
Listening distance will change the sound, as it does with any type of speaker. The farther away, the more room sound is mixed in and the higher the distortion you hear. You can sit closer, or treat the reflecting surfaces with diffusion or absorption. The controlled dispersion horn limits the width angle of the sound so you have less room noise compared to direct radiation speakers which have uncontrolled dispersion. Some freqs are wide, some are narrow, making a uneven power response which affects your perception of FR.
Have you watched the youtubes about the power response demo of these 305s? The mic moves across the horizontal but the tone barely changes, unlike the competitor's direct rad. speakers whose tone changes noticeably. This is why you can toe them inward to cross in front of you to reduce sidewall reflections, without screwing up the tone quality. More than any other horn, they sound the same on axis as off.
The "Image Control Waveguide" horn is the big innovation here, allowing the efficiency and directivity control of a horn, with minimal high order modes to color the tone quality as is common in traditional horns. A new horn shape from JBL is rare and big news. They benefit tone quality even further in the 3 series by using a standard tweeter in lieu of compression driver. Idk if the 7 series uses dome or CD tweeter.