Typically soundstage is a factor of how well a headphone does treble. The bass cues for your hearing end up being somewhat one dimensional. The treble is what gives a good sense of soundstage. This is primarily due to its clarity, wavelength, and the ears sensitivity to treble more than bass.
"Evaluation for low frequencies
For frequencies below 800 Hz, the dimensions of the head (ear distance 21.5 cm, corresponding to an interaural time delay of 625 µs), are smaller than the half
wavelength of the sound waves. So the auditory system can determine phase delays between both ears without confusion. Interaural level differences are very low in this frequency range, especially below about 200 Hz, so a precise evaluation of the input direction is nearly impossible on the basis of level differences alone. As the frequency drops below 80 Hz it becomes difficult or impossible to use either time difference or level difference to determine a sound's lateral source, because the phase difference between the ears becomes too small for a directional evaluation."
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http://en.wikipedia.org/wiki/Sound_localization
Basically this means that the lower the frequency the harder it is for the ears to be able to hear any timing differences in the soundwaves. If you remember that frequency is on the order of the inverse of wavelength. So the lower the frequency ( bass ) the longer the wavelength. Our ears will have trouble determining where the sound is coming from along with other phase cues for bass.
It is possible that more bass be it perceived or otherwise produced can mask the treble. If the treble decreases, which is going to give you a sense of more bass, then you are losing phase cues. These lost phase cues may shrink the soundstage slightly. The biggest source for lost soundstage cues besides the speaker/headphone is the quality of the music / sound source. If it simply does not have the cues in it then you cannot possible help it along with your headphones. If you have, on the other hand, a great recording filled with phase cues like a binaural recording, then the rest of your gear will be the limit. A DAC can be a limit if it is limiting the frequencies in the D to A process. This can be due to undersampling, noise, or other signal process problems. The same goes for an amp and of course the speaker system.
I would say if you hear more bass and less treble then you are probably losing some of the cues for position and phase. This should decrease the size of the soundstage a bit. How much is going to be up to your ears. Have you tried listening to some binaural recordings?
You can try "Beep Beep Love" recorded using binaural techniques here:
http://www.youtube.com/watch?v=FR9LoqwGTbQ
It does have some swearing in it, but it is mostly clean and the swearing is hard to decipher.
