Well I ordered the NVX should be here tomorrow. I went ahead and bridged the arc on the JL's and all I can say is WOW! That's what these components needed, more power. Sounds better than ever.
Can't wait to install the NVX on the focals now.
Higher sensitivity speakers will get you more volume from that amp but chances are you will lose low end. If you keep those JL's bridge the Arc 600.4 to then and add a small amp bridged to power your rear. The NVX mini fits well on the BT bracket and is only a 150.00 investment. Or you could do a combination of pa speakers and horns up front of the arc and some bass friendly speakers bridged off the NVX in the back. I'd start with a small second amp and see if the additional power is enough.