The Acute Classic is a replacement for the Acute 3. CD transport & DAC. The Acute 4 is a SACD transport that needs to be married to the EAR Dac 4.
I stated that I loved the sound of the Acute 3, but only felt the DAC is a little dated.The tube and transformer section of these designs is pretty timeless but DAC chips/circuits technology seem like they evolve everyday. I think for CD playback, the Acute 3 was already pretty good. If you are never going to play CDs, the Dac 4 or something else is probably better. Just watch out though, as there are some super high end DACs out there that do 384Khz or even 768KHz but some of them sound really poor so specs aren't everything from my experience. Plus I don't actually own any music of those resolution that aren't just sample files.