I don't know about those ones but usually Sonneschein are gel. There seem to be at least 2 types of VRLA with different voltage requirements. These may be the type with the low voltage, the higher voltage ones which are not gel can be equalised carefully. I don't think the gel can.
You can't do any good as you suggest with a lamp, you need to raise the low voltage string, not lower the high one as that would just be discharging the better ones.
Possibly the only thing you can do is separate the cells and charge each one with the maximum charge alowed for that type of cell ( should get that from Sonneschein).
This way each cell will see that maximum. In a string the high ones will dominate and reduce the volts available for the low ones.
With other types of battery equalising is done by applying an over voltage to the string, this overcharges the good cells but lets the voltage rise higher on the poor ones to get them back up.
With a string of gels you can't do this as the good ones will not tolerate the over charge.
It is a bit drastic but the only real way to find out what is happening is to do a discharge test on one of the highest voltage cells. If that came out fine you could do the same with the lower ones, if they are poor you could try to improve them by charging individually.
If the load test on the good one showed low capacity then there is little you can do as the whole bank will be tired.
Flux