The Rokits are powered studio monitors but I think are designed for +4db signals which might explain the loss of volume (as the PX-5S is -10db). They do have their own gain knob on the back which would help to compensate. I haven't messed with this as I use the Rokits for other applications and so hard to find one setting that works best.
The line in for inputs 3/4 on the Focusrite has two settings for high/low gain. The high gain can clip easily and the low gain is not enough. Once I change the volume on the PX-5S I can start to dial it all in and sounds good.
The pre-amps inputs 1/2 are still the best option on the Focusrite.