Arg, no the 2600 doesn't set separate crossovers for separate speaker groups; only some HK and Onkyo models have that feature.
As to the impedance setting on the 2600(or any receiver), set it at 8 ohms regardless of whether the specified inpedance of the speakers is 8,6 or 4 ohms. The switch or setting on various units for a lower impedance reduces the maximum voltage available from the rails in the power supply section. Because of Ohm's Law, when less voltage, which is the key factor in producing power, is available, less current is available(I=E/R; current equals voltage divided by resistance or impedance)and less power is available(P=VxI; power equals voltage times current). Although using a lower impedance setting lessens the chance of a unit starting to overheat and having its protective circuits shut it down, this is rarely a problem, even with 4 ohm speakers and should only be used as a last resort. Manufacturers put such suggestions in their manuals for liability reasons with units that don't have UL certification at less than 8 ohms without it, but regardless of that the 8 ohm setting should be used to preserve the maximum power capability.
