My sense is that Liz and Marakanetz may be making a fair point. I used to own the ARC Ref CD-7 with upgraded power supply. Some called it the Ref CD-7.5. I recently bought the Ref CD-8. To my untrained eye, the innards of the CD-7 and CD-8 look very similar. The main difference is the DAC IC. The CD-8 uses a Burr Brown DAC. The CD-7 uses another type of DAC -- I forget the name. To my old ears, the CD-8 simply sounds better than the CD-7. I found the CD-7 to be bottom heavy and thick. The CD-8, by contrast, has more air. The bottom end is less viscous than the CD-7. I attribute the difference to the DAC -- not the guts. Can't speak to other ARC DACs, like the DAC 8, but I think I make my point. It may very well be that in the case of a DAC or CDP source device, the DAC IC is the main driver of sound quality versus a SS or tube gain stage -- don't know. But I can report that the DAC in the Ref CD-8 sounds better than the DAC in the CD-7. My sense is that the innards are less critical. FWIW and IMO.
An interesting comparison might be how the ARC CD-5 compares to the REF CD-8. Both use the same DAC. However, I believe the CD-5 is pretty much all SS versus the CD-8 which has a tube gain stage and tube regulated power supply. If anyone has critically compared the two CDPs, please chime in.