Might be a good idea to just take it in stages.
1 ,That VR400 amp is unusual, only has line in, and line out, plus speaker connections. Check that it works by plugging in speaker, and use a pedal ( something like a Sansamp ) to get enough signal from your bass to make sure it works. Never run this amp without speaker connected.
2, The ah600 is a dual channel amp, needs speakers connected to both channels. try it with your bass , as normal. Personally, I would not run that amp without speakers , JIC.
3, Then , when both are working, I would try to link them.
My memory of the AH600 is vague ( although I could look at mine tomoz ), but I seem to remember there were various options to get a signal out, DI out, line out and effects loop. take a pic of the back of your amp, let's see what the possibilities are for you.
Mind you, if you get all that working, you could play the 02 arena,