Few things are as frustrating as having perfect background vocal recordings that don't "sit" well in the mix. If you push the volume up, they start to mask the lead vocals, but if you pull them back, they fade away and don't have the desired impact on the song. This is a case where judicious use of EQ can be exactly what the mix doctor ordered.
Start by routing all of your background vocal tracks to the same stereo auxiliary channel strip in your DAW, so you can use one instance of an EQ plug-in on all of them. Begin by listening for "hot" frequencies that are jumping out; a bit of corrective EQ to reduce those spiking frequencies will go a long way toward evening things out.
Next, try using both high- and low-shelf filters to filter out some of both the high and low end of the background vocals. It might seem counterintuitive to shave off both the highs and lows. But you'll find that once you have, you'll be able to mix the background vocals higher while still maintaining a "background" feel. Removing some high end helps the background vocals not get in front of the lead vocals, and removing some low end means they won't muddy up the mix.
The exact frequencies to shelve and amounts to cut depend on the project, of course, but if you find your background vocals hard to mix, give this trick a try!
These days more often referred to as simply "500 series," A physical device format created by API that allows various hardware modules to be mounted into a master chassis, which provides the power supply, input and output connections, and in some cases other features such as mixing, sidechains, and more. The 500 series format includes modules such as mic preamps, direct boxes, graphic and parametric equalizers, compressor/limiters, and more, from a wide variety of manufacturers. The 500 series offers advantages such as reduced price, increased portability, standardized design, compact form factor, and more.