I hadn't tried it with the passthrough open before, buy I just did.
It doesn't really affect the sound too much, although it does make it a bit louder.
IMO, the Basslink matches well with rock/heavy metal, as it can put out fairly tight bass. It can also provide boomy bass, if that is your preference. It is probably not as tight as a sealed box 8" or as boomy as a ported box 12", but I think it does a great job and I am very pleased with it.