To clarify:

When the empeg says "high level", it means 4v line-level, i.e., of the two line-level standards (1v and 4v), it's the higher of the two.

When the amplifier manual says "high level" it is using the term incorrectly. Perhaps they meant to say "high power" instead. They are talking about already-amplified speaker-level outputs. For example, connecting the speaker level outputs from a car stereo into the amplifier's speaker level inputs. Do not do this. Use the amplifier's line-level inputs, what it calls "low level" in its manual.