If even 10kg is making your lower back sore, my guestimation is that you aren't using your core properly, so all the torque is taken up by the erectors. If you aren't doing this already, learn to draw your belly and your balls into your body with your chest up -- this will help to activate and control your transverse abdominis, which is directly involved in supporting the spine.
On SteveP and Silverback:
1. Most beginners will loose core tightness and round out the back if they think at all about bending over, whereas pushing the hips back tends not to have this effect. Ultimately, to get the full ROM you will need to fold at the hips, not just push them back, but focusing on pushing back rather than bending over is a good way to start learning to keep the core tight and chest up while bending at the hips, keeping the movement out of the trunk. So to some degree you're both right.
2. As it stands, it sounds like OP probably doesn't have the problem of needing to go deeper than the floor will allow, so deficit blocks/steps probably aren't beneficial at this point. Once OP has solid core control, form and technique, and his flexibility is such that he can keep solid core control, form and technique without much of a stretch while the bar is on the ground, then it's time to go for deficits to increase the depth of the stretch under load.
In addition to pushing the hips back on the eccentric, drive them forwards and drive your shoulders back on the concentric. All this while maintaining rigid core control. Screwing your feet into the ground and pushing your knees apart (while keeping them back over/behind your heels) will further activate your glutes and help support your spine.