Integrated audio DSP with free tools?
Answering to myself, I found the perfect part for this, Analog Devices' ADAU1701. It does exactly what I need. It's cheap, really easy to program, and the software is free. All that is required is a 12.288MHz crystal and an EEPROM. It can even handle rotary encoders or potentiometers for volume control, or even pushbuttons. It can be controlled via I2C or SPI. It includes all the necessary DACs (four!) and ADCs (2). It's the perfect device for turning my stereo signal into 2.1, bass-boost it, LPF, control its volume, and much more.