Different amps perform fundamentally differently; listen for yourself if you don't believe it.
To your second question; No, you can't make a DAC confer the attributes of a better amp, and vice versa. If you want to upgrade the system as if different amp and DAC, you need a different amp and DAC. There are no shortcuts to superior systems. There may be different configurations - but not shortcuts.
Don't get hung up on distortion specs; it's a great way to build a mediocre rig.
To your second question; No, you can't make a DAC confer the attributes of a better amp, and vice versa. If you want to upgrade the system as if different amp and DAC, you need a different amp and DAC. There are no shortcuts to superior systems. There may be different configurations - but not shortcuts.
Don't get hung up on distortion specs; it's a great way to build a mediocre rig.