First, the CB3 does not have DIRAC room control. I think it may be complicated to setup because the CB3 has all these different crossover points and crossover slopes/types. If I remember right, you can actually set your subwoofer crossover frequency different than you rmain speakers, and use a completely differenct crossover type (such as Butterworth vs. Linkwitz-Riley).
Since the CB4 needs to support DIRAC room correction, they got rid of all that complex crossover options and just went with the industry standard where you set the crossover point for each speaker and anything under that point is automatically sent to the subwoofers. The CB4 upgrade is primarily the DSP board and the HDMI 2.0 board (which support 4K passthrough). The CB4 also supports DSP and DIRAC processing at 96khz (it even sends 96khz to the DAC boards). The CB3 only support 48khz processing, if I remember right. The CB4 supports all DAC boards compatible with the CB3. The firmware on the DAC boards needs to be updated also to support CB4 DSP board.
The CB4a (new model) supports Dolby Atmos and the new Xtreme 3 DAC board, which has 6 output channels. The CB4a supports up to 12 output channels. It also supports HDR over 4K.
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As far as sound quality, it's all Theta sonic signature. The Theta DSP does weird things to the waveforms when it processes in DSP. I think they have some sort of waveform smoothing, which they rave about. To me, the Theta sounds bright and sterile. I don't know if it's this weird processing or the design of the DAC boards or what, but it's not my preference.