The KH310II and KH350 are included in the new Dolby
DARDT tool which is useful even for home setups to figure out what sort of speakers will be suitable for your room and proper placement.
310II is 118 dB and 123 dB Bass Managed
350 is 120 dB and 124 dB Bass Managed
KH420 is 117 dB and 122 dB Bass Managed - so the new 310II is higher!
Though, note the KH120ii/150 Bass Managed numbers:
120ii Bass Managed is 119 dB
150ii Bass Managed is 120 dB
So at these suggested prices it's perhaps a smarter idea to look into one or two KH750's and a pair of KH120ii's or 150's which are roughly the same Dolby DARDT SPL volumes as the KH350's alone and will likely go lower in frequency response anyways as the KH750's go below 20hz.
One thing I would be hesitant about at least in a studio setting is the lack of metal grills. The KH750/120ii's and 150's metal grills are really useful.
As careful as someone can be in a recording studio, you never know if a flying drumstick or someone walking around with a mic stand or even swiveling in a chair with a bass guitar is going to hit your speaker. I remember taking the KH310's out of the box and it was quite difficult to not accidentally hit the midrange as it sticks out so far and is unprotected. I forgot to mention kids/pets: I've had someone bring over their child and within 5 minutes they managed to knock over a heavy/sturdy floor standing speaker into a wall and damage it.
The old KH310's and 420's do have optional grills which is nice, but they aren't cheap and not as nice looking as the KH120ii/150/750 grills. I would hope there are optional grills for the 310/350 also at least for the people who may want them.