questions about streaming source for a headphone amp

I've owned some decent headphones in the past but I want to move up in the headphone world (Clear, 800s, Arya and a suitable amp). Basically, I'm trying to figure out if getting good headphones means building an entirely new system. I've been researching headphones and amps, and when I came up for air I realized I need a suitable source....

In the past I've plugged my headphones (e.g., Sennheiser hd600, Grado RS2e, etc) directly into my phone or laptop or preamp, and it's been fine. I'm guessing, though, that something like the 800s needs a dedicated amp. I'll mostly be streaming music and I'm not especially keen on buying a new streamer and dac as well.

1) Would it make sense to do this, using my current stereo system: streamer > dac > (preamp?) > headphone amp? My preamp has two sets of "outs"; my dac has both balanced and unbalanced outs. I have no idea if I can run cables from either the preamp or dac in two separate directions. I know this is newbie stuff but would I have to turn off my speakers (at the amp or preamp)?

2) I'd like the source to be consistent in quality with the headphones and amp, the whole weakest link thing. I know this gets asked all the time but is an iPhone or MacBook an appropriate hq source for a good heaphone/amp combination? In that case, to stream hi-res, would I use an external dac (Audioquest dragonfly??). It feels a bit strange to me to spend $$$ on a headphone/amp combination and then stream the music from an iPhone via a portable dac. 

I'm interested that there aren't more headphone amp/streamer combinations. The Naim Uniti Atom HE looks cool, but not inexpensive. (The Aurender A30 ... $$$.) 

In short, how do people stream music through a headphone amp? Do you simply have a dedicated streamer and, if necessary, dac? (My son has a bluesound node I could experiment with.) Do you use your phone or laptop through an outboard dac? I'm sure I'm missing all kinds of things here... 

Thanks!