In Direct or Stereo mode, you can use high rate out to the R/F's if they are 24/96 and can still use a sub (in any other mode you need std. rate to all the speakers if your mixed).

There is a difference between HS output or not - without 24/96 and using a normal 16/44 signal the sound has more pace and immediacy with more bass slam, a more forward presentation.

With HS = Y (24/96) the bass deepens, the bass slam is less a slighty more recessed and smoother sound. I turn it on or off depending what I'm listening to. The difference is not night and day.

Not sure if the subtlety of this will come thru in cinema or with all channals being used for music and all 24/96 - I only have the experience in plain old vanilla 2 channal not with Trifield etc.

*I* would far rather upgrade my Fronts and centres than rears as I listen to a lot of music .. but your mileage may vary especially if you are HT oriented.

Roon/Tidal, SBT, Trinnov ST2 , Twin Devialet D premiers, Vivid Audio Giya G1 Spirits, dedicated and treated room.
Meridian gear: 568.2mm and DSP5000's acting as TV speakers.