Yes you can. The Element II RCA Out are a preamp, so you can use it to control powered Monitors like the JBL. The power button also acts as a switch between the two, you make a long press (hold for about 2s) and it switches between headphone out and preamp out.
Just be careful as I think the Element I didn’t have pre outs, so double check if you’re buying used. Element II is regularly with B-stock in JDS Labs page, it’s also a little cost save.
I’ve had mine for about a Year now and I am very happy with it.
I’ve had an EL Amp II for ~ 4 months and can say the same. The preamp works exactly as described and it makes switching between powered speakers and headphones incredibly easy.