How can I get the channel subassembly to attach to the assembly insertion point at the channel flowline marker? I need the flowline to follow my profile elevations.
Try this: attach the channel subassembly to the assembly center marker normally. The flowline should be some distance below the assembly center marker, depending on your specifications. Next, use the Autocad MOVE command to move the channel subassembly vertically up the distance from the assembly center marker to the channel flowline.
The assembly should now behave as you want.
I just tried it and it worked great. Did you attach it first normally, at the point code channel top? Then use the normal, regular, run-of-the-mill Autocad MOVE command (NOT the Move command associated with assembly construction) to move it up? It should work easily.