There is also another opinion that what is lost at source cannot be made up later on in the chain.
This argument is trotted out perenially as if by rote by those arguing 'source first', but is a meaningless statement as you also cannot get out what any component doesn't pass through, which includes amp and speakers, the weakest links in the system limiting what you hear, whichever components they may be. Against that the primary argument for a major focus on the speakers is because they most influence the character of the sound - but that is not blindly saying 'speakers first' not least because of course the very behaviour of speakers can be adversely affected by an amp not up to the job. Overall considerations need to take into account the effect of all components, though that may be moderated by any upgrade intent, especially in the short term when limitations might be more acceptable in the promise of realisation of the full potential soon.
Curiously the one valid argument for strong consideration to be had for the source is rarely mentioned: namely that if any component modifies the signal in any way, e,g. limiting, enhancing colouring or distorting (whether spectral range, dynamics or timing etc), that modification is carried through by the subsequent components, each then superimposing its own 'signature', however more or less significant that might be: so a bad source will sound bad however good the speakers are. However, with modern digital sources it seems that even quite low down the cost spectrum sources may not be bad as such, just not as good as others, while as with everything the law of diminishing returns comes into play, and available budget, especially If limited, presents a challenge as to where most benefit arises to the intended listener.
in this case, however, the OP has the speakers and power amp, so the question is simply one of what source and preamp within his budget are best (bearing in mind that some DACs do not need a separate preamp if analogue sources aren't used)