Good questions. So here's the quick facts.
The NFB-11 and R2R-11 are decently powerful, decently priced, units. Both offer single ended output only, but have a myriad of inputs, line/pre-outs for using with other Amps or powered monitors respectively. By all accounts they sound great. The NFB uses a delta sigma dac, and the R2R uses a, you guessed it, Resistor ladder dac. Different sounds, Audio-GD recommends going for the NFB-11 if you're going for your first product.
The Aune X1s and X7s are both fine units. However, the 10th Anniversary Ed of the X1s has a more powerful built in amp than the one Zeos reviewed but it's DAC is considered older and less high end these days. Personally, there's DACs I think are more resolving, tho whether I can hear it or it's confirmation bias is up for debate. The X7s offers a balanced output via 4-pin XLR and is Class-A (which is better, I guess?), but because it isn't balanced inputs, and it uses a Phase Split, it's not considered fully balanced.
The EL Stack by all accounts is good, and offers some power to just about any headphones. But is also not balanced. Though it's amp does have a lower distortion. Whether or not you could hear it is up for debate since it's something like point 003 or something, and the EL DAC is like point 0003 percent. So that's a thing.
So, all that said. The EL Stack is probably amazing. The Aune Stack is alright, but there may be better options than the X1s for the DAC section, and the X7s isn't fully balanced. The NFB and R2R are both by all accounts fine choices as well.
For a while, Z used the Jotunheim, because it's fully balanced, class A and has a great Delta-sigma dac with a Multibit option.
He is also using the Aune S7 balanced amp, but he said the EL Amp offers similar power for less money and easier availability so... Yeah.
Edit/Note: if you can get one from the next drop, the THX AAA 789 is a fantastic amp for the money. That's from personal experience too.
Edit/Update: the Monoprice Liquid Spark and JDS Labs Atom are also fantastically measuring Amps for ~$100. Can pair them with an SDAC for the same price as the X1s (as of March '19). Of course, that's a stack and not an all in one. So that's also something to consider.
Anyways, if you have any more questions, feel free to ask. Good luck.