I've owned the Jot as my primary Solid State amp for several months now. To respond to your third question, it's definitely a good enough amp to bring out the quality of the other devices in the chain, DAC included.
While you'll hear people who are certain they can hear the difference between DACs, my experience is that, short of fixing some problem in the implementation of another DAC that introduces audible noise, upgrading a DAC provides some of the least bang for the buck in the audio chain. Lots of blind A/B tests come to the same conclusion. The reason to upgrade a DAC is really its features, rather than sound quality.
I think your decision to go with the least expensive option in your list is a good call. If you didn't try it, you'd always have a nagging doubt, and this way you'll be able to test for yourself and draw your own conclusions - always the most valuable kind.