Without having a dedicated source for each speakers and headphones, and without dual XLR outputs on your sources or preamp, you're going to be making a sacrifice (i.e. at a minimum an extra connection) for one or the other.
I'm not sure I'd ever want to run a preamp into a headphone amp (which has its own volume control), though I know some do that with a tube pre to fatten up the sound, and if you need input switching to a common set of sources then that pushes the point.
Lots of balanced headphones amps do have an XLR loop-output, but the downside is that you inject and extra set of connections before your amp & speakers. And since most audiophile have much more invested in speakers, that may not be an acceptable compromise.
I think the better scenario for your setup is to make use of the existing RCA output, and massage it to work for your new balanced headphone amp. A few balanced headphone amps have differential inputs that can take a single-ended input with NO performance penalty - examples of this include the upcoming new Headamp GS-X mini, and almost any Stax amp. If you get one of these amps, you're golden! Other balanced amps cannot do this, and require a balanced input for best performance - examples of this are the older Headamp GS-X (Mk I and Mk II) amps, and the Headroom Balanced amps. If you get one of the latter amps, an option is to insert a Jensen ISOMAX transformer that will convert and SE signal to balanced XLR. I've used these to good effect. It's better to not have to use them, but I've heard them improve performance for amps like the Headrooms and older Headamps that would otherwise have been handicapped by a SE source.
BTW there is a Headroom Balanced Home (Max modules) on US Audio Mart for an absolutely screaming deal right now ($625). Beautiful sounding balanced amp that sounds like it costs $4K; I still enjoy mine on occasion with Sennheisers despite also running a top-tier $20K Stax setup for my main headphone rig. This Headroom amp also pairs superbly with Focal Utopias. Also pick up some Jensen ISO-MAX for like $225, and you can be done with a top-tier headphone amp for so little cash.
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