So from what I understood, top of the line Iems/Ciems won't really benefit from the amp part, but will the tube DAC make a significant difference?
I don't plan on buying a headset so I'm trying to figure if this beautiful thing would be useful appart from shining on my desk (which is already something!)?
No, no, that's not what I meant. What I mean is that you'll want a better amp section, ideally with <1 ohm output impedance and low gain for totl iems/ciems. Those usually make use of multiple balanced armature drivers, which utilize complex crossovers to put together the sound. Because of how BA drivers work, they have varying impedances over different frequencies, so ideally you'll want an amp with <1 output impedance so that it won't affect the iem's sound signature. The general guideline is that you'll want the output impedance of your amp to be less than 1/8 of the headphone's impedance load.
The output impedance of the Aune T1 is 10ohms, so I would not get the Aune T1 for any IEM utilizing BA drivers (in your case, the UM3X-RC) unless you hook up the T1 to a seperate amp. The Schiit magni would be the best desktop amp out of the aune, o2, and magni for IEMs.
As for the tube part of the Aune T1, I don't know how much of a difference it will make in reality. You should probably read up on some tube-rolling impressions with the T1 to get an idea. The dac itself does make a difference if you're using the onboard dac of your motherboard or laptop though.
I've really been eyeballing this deal. Would my AD700s benefit? I know they're not top of the line phones, but they're all I have. And if I could get at least some minor positive experience out of the setup, it might convince me to buy some new ones.
Even though the AD700 is rated at 32ohms impedance, which is less than ideal for the Aune T1, it will be fine because the T1 is rated for driving 30-600ohm loads and the impedance phase of the AD700 is fairly linear, so its sound signature will not be impacted much by the Aune T1's 10 ohm output impedance. It will be a good entry level amp/dac that will introduce you to better sound quality.