Right. So if you hooked up an 810 or 215 or whatever (most are 4 ohm) with a single input to the 7000, you'd only have 1 cable connected to 1 of the amps and it'd be the exact same as setting your 3500 on one of those cabs.
If you mean the XL series HArtke's with the stereo/mono switch and two input jacks - no, they don't cut off half the speakers to achieve 8 ohm and add the other(s)s for 4. Obviously you could have 2 speakers on each "side", just use 8 ohm speakers ran in series on the magenta and blue wires for a 4 speaker cab. In mono position the right hand jack can be used with another 8 ohm cab to achieve 4 ohms total impedance.