Don't forget that with the Schiit stack you still need a source. Something like the M11 is an all in one DAP. A fairer comparison would be something like the Chord Mojo. I happen to have that as well as the Magni/Modi and from my testing I could hear a but more warmth from the Mojo.
As far as value, I think the Mojo coming in at nearly double the price of the stack brings portability to the table, obviously, but also a fairly powerful DAC, using Chord's proprietary FPGA implementation.
You have to look at each system's subsystems individually here.
Source: M11 has the storage and interface; the Schiit stack needs something else to store and feed digital audio into the Modi3. In terms of money, this accounts for most of what you pay for on a portable player.
DAC: I wouldn't sweat the differences. Maybe five years ago the line output voltage was lower than AC powered units, but not anymore.
Amp: A tiny circuit board running off a battery despite being well designed wouldn't be able to compete with a competently designed circuit running off wall power, even if it uses a rather generic power brick, which has about 10X the power output without any penalties to noise and distortion (other than exposure to bad power from the walls). That said, this is highly relative. If you use a high sensitivity in-ear monitor, even a good DAP can be overkill in terms of sheer output power, without any issues on current and voltage.