SACD Player Mods vs. Outboard D/A converter?

Geez me too, somewhat...

I just took delivery of a used 555ES that I can't play until my new amp comes in a few weeks and I'm glad I didn't release my CI Dac yet anticipating that the stock 555ES redbook may be something like my original 333ES was (which I had "Kerned").

I can say that after Richard modded my 333ES the redbook jumped substantially forward in musicality (hackneyed term I know). The SACD section was better but not like the redbook was. Based on that experience I am prepared to leave my 555ES stock for SACD and use it for a transport for my DAC (which I dearly love at its pricepoint).

Maintains the Sony ES warranty, gives me what I am hoping for is pretty good SACD (based on stock 333ES sacd in my past) and gives me a good bang for the buck - not nearly the best mind you but enough for now (or until I talk myself into sending it back to Oregon...)

Get a Chord DAC 64 and any transport jumps levels in musical presentation. I have one and use an Electrocompaniet, a Pioneer 300 disc changer, and a Sony 333 changer as transports. All sound great through it with no mods.

Get a Marantz.

Keep in mind, going the mod route saves you the added expense and anxiety of a digital cable and a second power cord. (Anxiety because you have to worry about whether you have made the best choices. Some of us like the angst, though.)

My impression is that most of the mods are strictly parts upgrades. This raises a fundamental question in my mind: What is the relative importance of parts quality vs. circuit design? Of course they both matter, but what I am asking is, can you take the basic Sony (or other brand) circuitry and get near state-of-the-art performance merely by upgrading the parts? Or is a different circuit design needed? If I had any engineering background, I might have an informed opinion on this question.