If you get the Modi2 or Modi2 Uber, you will NEED to also buy the Magni 2 or another standalone amp, since the Modi is only a DAC and has RCA outputs with no volume control.
As far as increasing soundstage and bass, over what? These two little guys are a massive step up over motherboard connections on a PC or even a PCI sound card. So they will in all likelyhood sound better than your current setup if you don't have a dedicated headphone DAC/amp rig.
If you're looking for a guaranteed bass increase, I think ifI has a few amps with hardware based bass boost functionality.
I have Fiio E17k and planning to get K5 as a Dock. Which one will be better?
If schiit is better, I can sell my current e17k and get the Stack