Headphone DAC for under $150.

Greetings,

I am a high school violinist looking to assemble a budget mid-fi system. I currently have a Primephonic subscription on an iPad (so plenty of lossless music available) and Sennheiser HD569 headphones. Because of the low impedance of the HD569s, I don’t think that I need an amp/preamp.
I have initially narrowed my options down to these three DACs (in no particular order):

1. FiiO Q1 Mark II 
2. Schiit modi 3
3. iFi Nano iDSD Black Label

(As far as I know, all three DACs have USB inputs that are compatible with both iOS devices and PCs.)
So here are my questions:
1. The iFi is almost twice the price of the other two; is buying one used a significant upgrade over a new $99 DAC?
2. Is there any sound quality difference between the FiiO and the Schiit? Right now I don’t have access to any DSD files, so is it better to buy a PCM-only DAC?
3. I have read on several other threads about the Apple Lighting Camera adapter. Would I need to purchase that in addition to any of these DACs? (My understanding is that the FiiO comes with a specific Lighting to micro USB cable.)
4. If there is something that I’m missing (either a product or a concept) I would appreciate it if you would let me know. 

Thanks for your time and knowledge!