*IMO*.... add a couple of ac110's .. these guys love "bigger is better" mentality in filtering.. it isn't always so .. you DO want some redundancy but huge amounts of bio media isn't required to run tanks efficiently.. once you add the 110's that should have you covered for any reasonable stocking levels.. water changes are the key for keeping tanks clean, *not* more filters...watch your parameters as you stock .. *if* your ammonia never has a long term rise (more than a day or two.. with proper stocking you shouldn't even have a spike really) your good.. period.. adding *media* is the fix for ammonia spikes , not *adding filters* per se...adding the 110's will give you plenty of flow (in theory, depending on tank scape) to keep detritus from building up undergravel to great extent..in your case the fx6(s) alone should have room for enough media to keep a reasonably stocked 300+ gallon tank cycled the ac110's will take up any "slack".. maintenance is the key , keep the water changes consistent and large .. vac when necessary .. clean out a(1) filter about once a month...it isn't as tough as some people make it.. enjoy the hobby...
