"High" and "Low" refers to digital signal levels, 1's and 0's so to speak. The respective measured voltages will depend on the supply and the general circuit layout and may differ from one circuit to another.
An oscilloscope or data logger is required here,
especially if you need to measure and check the timing at two or more points simultaneously.
B&O's circuits may seem fairly simple at first glance but they can indeed be tricky. Servicing the electronics of a Beogram 4000 is not the easiest thing and, with all due respect, unless you feel absolutely safe I would recommend leaving servicing to someone who knows these beautiful machines and the way around their pitfalls.
Having said that, I would start by checking the transistors in the H-bridge, 8 in total I think and eventually work my way backwards.
Martin