I'm trying to figure out why you would have a cold spot down the center of your back laying on pads in the pad pocket? Could it be the pads are not allowing your back to settle right into the center, with the pads forced into a "U" shape and your weight supported more by the outside edges of hammock and pads, so that there is a space between your spine and the pads? I don't really have an idea, just guessing wild here. I have not noticed this in the JRB Bridge, don'y yet have experience with the WB bridge.
When you ask "Without using a special under-quilt - just using a pad in the undercover (of the HH) or sleeve (of the WB) - I wonder what other's experiences are.", I'm not certain what your asking?
My experience is that I have been warm in the 30s or a little lower in the HHSS with 1 pad and space blanket. I also have something called HH kidney and Torso pads ( not many folks seem to have those ) which greatly increase warmth and also add down or fleece jackets in the UC under the pads, which gives another amazing warmth boost.
I have also been plenty warm and comfy in a JRB bridge with a torso sized piece of WM Blue pad in the pad pocket of the JRB bridge, but have not used a pad for winter temps, just an UQ.
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.