The 1818VSL nor the Studio 192 will work as a stand alone preamp. These must be connected to a computer to be used.
You can use the Digimax DP-88 with the UA Apollo however there is a small issue with that configuration.
For 44.1K and 48K operation, the DP88 is a good pair with the UA Apollo, you can use the MIDI Ports to control the pre-amps with the DP88. However for SMUX operation at 88.2K and 96K where you only get channels 1-4 due to the higher sample rate, the UA Apollo must be Wordlock Slave to the DP88. UA is aware of this issue.