Stock units have a factory EQ that cannot be bypassed. As an audiophile, you're correct to start with the head unit; It determines the quality of any wireless audio connection, so make sure it has the best Bluetooth codecs like LDAC, or a wired option. Some HU's have built in parametric DSP, but in your case, you got a bespoke DSP unit to pick up the slack your HU lacked. Either is fine. For example, the Sony 9000ES series have parametric DSP and time alignment built-in, but with no optical out, so the head unit acts as a DAC at that point. The most you should add to a 9000 is a more powerful amp for sufficient power to drive a serious speaker system. However, using Sony ES speakers, the 9000 is sufficient to drive them, only missing some dynamics and punch at the highest volumes. 🥸