Improvement needs to be described. What are you looking for. You can improve the sound of your preamp, dac and amp by using filter chokes, that will make more improvements to details, resolution, black background, dynamic swings, clearer sound, better tone, etc. Caps and resistors can be replaced to improve the overall sound also. You don't need to spend more money on upgrading to more expensive components if you understand what can improve the sound. Once you understand this, then you can find components that use the specific designs to improve the sound.
cost of improvement
Of course there are exceptions to this, but in general, does one usually have to spend twice the amount of the previous item to actually notice a tangible improvement? (Provided the equipment hasn't reached the law of diminishing returns). For example the difference between a $500 set of speakers or amp might not be a noticeable difference from a $650 set of speakers or amp. But by the time one jumps from that $500 price point to the $1000 point, does the improvement become apparent? My question is does it usually cost (1.5 times) (2 times) (4 times)? as much to reach each "next level" of component improvement until the law of diminishing returns sets in.