Emax

I'm pretty sure the emax doesn't have a latch/unlatch feature for 
playing loops. Your best bet would be to use a midi controller that 
would send a "note on" message with the first press and a "note off" 
message with the second press. This could then toggle the loop on and 
off.  You might want to look at a Peavey PC1600x for this.

Hope this helps
Steve