I'm not sure but I believe bass management only applies for the A channel mains, not B channel mains. I have to check this. My thinking is once you load both A & B channels, then bass management is off.
The RX-V1800 has a robust power supply and amp stage and should be able to drive both pairs full range. However, it depends on how big the room(s) are and how loud you want to drive them. Its still an AVR and it has limits. Before I moved I ran a 6.1 PSB Image series speaker setup full range while exercising and the AVR was able to crank the volume without clipping or getting harsh and strident.
I'll get back to you guys about the 1st paragraph.
Update...
There is no channel A/B for the mains. There are connections for just mains. However, there is a zone2/zone3 capability with zone2 using the "presence zone2/zone3 speaker terminals in the back. I will still need to check if zone2 will operate with bass management setting of the main zone.
Update....
On page 114 of the manual, it states that inputs are shared across all 3 zones and you cannot use the same input across all 3 zones simulataeously. As eg, if the main zone is using DVD input, its unavailable for zone2/zone3 at the same time.