My own answer: Both
Within reason, naturally. But . This question has truly, no right or wrong answer, as everything matters. The How much part depends on way too many variables to be resolved quite so simplistically.
I wanted/have a system which uses multipel sources. So a pretty good pre is in order. Certainly so is each source. Radio, CDP/DAC, PC/DAC, etc.
I have a great preamp. One whose attributes accentuate the musical side of the ledger more than the articulate side however, it is most revelatory in exposing errant signal info as well so most sources I own, though not commensurate or on par with my preamp, do engender adept signal generation. Consequently, my pre has an additive effect on improving things generally speaking. Ill put it another way. Regardless the amp Ive attached it too, and the source for that matter, the sound has stepped up every time.
It follows then that every step up I can attain in a source output, Im again improved upon with the final outcome.
Being practical about things as Im prone to do not having the ability to buy top tier devices whimsically or impulsively, I feel there needs to be a fairly good disparity in there somewhere for appreciable signal loss or more pertinently, degredation to occur or in fact, be realized.
For example, todays market is producing preamps that perform very well indeed for less than $4K MSRP. A few online rags have chosen some for under $3K as some of their best of 2009 items. Very good digital one boxes are had now for under $2K! Its taking a much higher level of investment now to out run the more modestly priced fare lately.
Knowing now what I know of that which I currently own, Ive got to say IF a pre is to be used as central unit to a rig, the pre investment will show itself as one where spending more (to a point) is a very good idea . More so than on the source again now, within reason. Perhaps its akin to the adage you wont want to run recaps on a Vette. .. and I say all this with the notion I believe strongly in building fromt to back rather than the speaker first concept as Ive found this approach pays off quicker.
BTW the MSRP on my preamp is higher than that of any other piece I own in my system and may well get a further investment via upgrades. So if nothing else is concluded from my exp, it can be said wherever you infuse a larger investment into components transmitting or developing the signal, it will pay off. The key then remains merely the matching of the components to one another. I do doubt seriously too, were the investment contrary to mine, and I had spent 4K on a source and 1K on a preamp, would I have had as good results or better? Id say No and not look back, if my DAC, or receivers preamps are providing me any true insight into this suggestion.
Find the magic in your affair where you can. In my own, its my preamp predominately, but not exclusively. Its the sum of the whole more so than just one piece.