mainly SQ because the higher the pre out voltage the more resistant to noise it'll be. but it can help SPL some too by making the amps have to work less to amplify the signals. keeping everything cooler.
its not gonna make any drastic night and day differences though.
IDK, Going from 2v preouts to the 6.5v preouts on my new 860mp was a huge difference. At high volumes the bass sounds noticibly clearer. I also think it gave me around 1-2 db of spl. Nothing very drastic. but definitely a very noticible improvement.