IMHO, the opamp should be considered first because it is the so-called "interpreter" of the player. a good cd transport is only limited (am not sure ) on how fast he can read cd formats while the DAC is simply the converter.
In a series of upgrades, opamp is the first one to be modified.
OK lets put it this way, the basic part of the CD player is the CD Transport & the DAC. That's why in a true high-end system hiwalay ang dalawang ito.
Mid-end set-up usually we are using CD player where both CD transport & DAC are already in the package. You cannot upgrade your transport because it is custom fit to any brand of players so you don't have any choice but to change/opgrade both OP-amp & power supply.
You are right me also believe that OP-amp is the 1st part to upgrade below are my list:
1. OP-amp
2. Power supply (Both the diodes & Capacitors)
3. All Capacitors must change with premium one
4. Resistors, change them with precission one
5. RCA connectors
Sir JojoD, John & Garp can you post your list of upgrades start from your priority.