You probably talk about BLT's Balanced Line Transmitters or BLD's balanced line drivers. If you amp has the capability.
Just a little hint you can use PS/2 computer extension cables or so calles 6 pin Mini DIN cables (mouse or keyboard), it works. Just threw 4 of them in at the weekend.
line voltage limiter.. it's called the volume knob. you can use a line driver or pre-amp and set the max voltage to your desired signal level like 4V.. then set the head unit volume and leave it, using the pre-amp volume as master from then on, too. this also eliminates head unit distortion.