After lots of listening, I've concluded that the 'flat' sound stage I hear from this DAP is due to the amp. In listening to Sgt Peppers 50th anniversary tracks (comparing to original stereo mix), I switched between all my rigs:
DAC mode configurations
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PC ==> Ultra-Fi ==> Shadow
PC ==> Ultra-Fi ==> Topping N5
PC ==> Cowon Plenue 2 (balanced out)
PC ==> Cowon Plenue 2 (SE out) ==> Shadow
PC ==> Cowon Plenue 2 (SE out) ==> Topping
PC ==> X5iii (balanced out)
PC ==> X5iii (line out) ==> Shadow
PC ==> X5iii (line out) ==> Topping
DAP mode configurations
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Cowon Plenue 2 (SE out)
Cowon Plenue 2 (balanced out)
Cowon Plenue 2 (SE out) ==> Shadow
Cowon Plenue 2 (SE out) ==> Topping
X5iii (SE out)
X5iii (balanced out)
X5iii (line out) ==> Shadow
X5iii (line out) ==> Topping
Transducers are Monolith M1060 & iBasso IT03 both with 2.5mm TRRS balanced connections. Use a 2.5mm TRRS to 3.5mm TRS adapter to listen to SE amps/outputs. Same source files on PC @ DAPs -- 24bit FLAC (down-sampled to 16bit for output to the Ultra-Fi which doesn't do 24bit).
Lots of interesting/subtle differences but the bottom line is there's less depth to the sound stage from the X5 amp (SE or balanced) and more depth when
something else is doing the amping.
Good news is that the X5 ==> Shadow configuration ties for top slot. It's a multi-way tie, as I can hear subtle differences between some of the configurations, but can't really say better/worse. Looking forward to see if the Fireye HDB competes with the Shadow (marginally preferred over the Topping, which seems
slightly bright).
If you're interested, here are my impressions of the new vs old mixes of Sgt. Peppers:
https://www.head-fi.org/f/threads/sgt-pepper-50th-anniversary-releases.851092/page-2#post-13525401