Although you can control the sub volume with it’s own volume control, you will have to adjust this every time you change your main volume. The sub volume control is mainly to adjust the "relative" volume levels, and once adjusted, usually is left alone. Tape monitor outputs are fixed and do not respond to main volume levels.
I would avoid line level splitters at all costs.
Most subs have a high level speaker input as well as a low level line input. I would use these tee’d off the main speaker outputs. Much better than a line level splitter IMHO.
Another trick I have used is a isolation transformer connected off the main speaker outputs. The transformer isolates the output speaker levels and reduces the level to line level for the sub. The product I have used is called the "ZDirect" made by ART. It has two -20 dB attenuators and an isolation transformer, so it can reduce speaker voltage levels to line input levels easily.
I would avoid line level splitters at all costs.
Most subs have a high level speaker input as well as a low level line input. I would use these tee’d off the main speaker outputs. Much better than a line level splitter IMHO.
Another trick I have used is a isolation transformer connected off the main speaker outputs. The transformer isolates the output speaker levels and reduces the level to line level for the sub. The product I have used is called the "ZDirect" made by ART. It has two -20 dB attenuators and an isolation transformer, so it can reduce speaker voltage levels to line input levels easily.