Honestly, it looks as though you should use both in tandem. I might be wrong but the EQS, being created as an EQ but usable as an OEM integration processor, lacks the ability to sum signals...
Also, neither unit has the Accubass feature, or the delay feature found in the DQ-61.
You need to use the LC6i for channel summing but the EQS also, to Tame that curvy sumnabich. The EQS does have that 40hz eq knob in the sub eq section... That's good.
If you look at your LC6i , does it have 3 sets of separate lines going to it? Or are any or all of the lines looped together? Also, are any of the summed indicators on the face of the unit lit in green? How many of the preamp outputs are you using?