Roger, how long are your runs of baseboard? Also when you bypassed the HX, are you still running the second pump for the oil burner to feed your zones? I'm assuming you just have 4 zone valves to heat those zones, with the indoor pump serving them? I noticed you turned up the inside pump earlier in addition to the outside pump. To get more heat out of the HX, you'd need to speed up the OWB pump and slow down the inside pump. Depending on zone temps and length of baseboard runs, you may be able to drop the inside pump back to 2 or even 1 to keep the temp up across the plate. Hard to say without looking at it so maybe I'm out in left field but that's what I'm seeing. May need to upsize the supply loop as well like you said, again dependent on load. 1" PEX is just slightly bigger than 3/4" copper and unless the house is pretty small, that's not enough to serve 4 zones of baseboard