Use a tube saturation plugin (like Analog Obsession BritPre ) to add 2-3% grit.
To get the Avalon sound, use the LALA (an opto-compressor similar to the one inside the 737) and follow it with the SSQ or Patron (for that smooth, musical EQ). avalon vt-737 plugin free download
Use a slow attack and medium release. You only want the needle to move about 2-3dB. Use a tube saturation plugin (like Analog Obsession
I can give you a specific to make your free plugins sound just like the $4,000 hardware. You only want the needle to move about 2-3dB
If you want a permanent tool for your kit that doesn't cost a dime, you can recreate the Avalon's "Tube + Opto + EQ" signal chain using these high-quality freebies: A. Analog Obsession: LALA & SSQ Analog Obsession is the king of free analog modeling.