While the statement It all starts with the source and anything lost from the source's signal will prevent your other components from reaching their full potential is true, it is also true that inferior downstream products (pre/amp, speakers) will prevent your source from reaching its full potential.
I tend to agree with Eslaudio related to digital sources. Technology related to digital sources change at a far greater pace than amps, pres, speakers (IMHO).
For example the VTL Compact 100s amps in my main rig are at least 30 years old and still going strong. Whereas my 10yr old Levinson 360s DAC, while still a great piece, has been outpaced by technology. 24/96 was once cutting edge but now we see sample rates for 24/192 with USB inputs, etc
Will the $300 Emotiva replace the $7000 Levinson, most likely not? But the choice also depends on how the stereo is going to be used, (i.e. critical listening or background / streaming music or CD transport). Are there other sources as well?
If you need to compromise on cost I would look at first getting the best speakers, integrated and wires you can afford and then look at the digital source. Cheap doesnt mean bad. Ive gotten some really good results with a dyi NOS DAC; not the end all in resolution but very musical.
So in the end its a matter of choice and there is no golden ratio on what to spend compared to other components.