The z-axis on the SX2 as standard is supported by a torsion spring and variable length arm - whatever height the head is the tension is wrong so you always have to have a hand on the feed lever as you unlock the head to stop it dropping - not very clever. Little machine shop in the states sells a kit which adds a gas strut to replace the spring - here's my version of this mod;
The strut wasn't cheap because I wanted a bespoke preload to exactly match the weight of the head but it was worth it as the z-axis is transformed - it doesn't move at all when you unlock the head wherever it is in the travel.