headphone amp confusion?

Yes, you would connect it to your source (in this case, your Essence ST)

I just took a sneaky google at the Essence ST and saw RCA connections.

You need a DAC (digital analogue converter) to convert your digital files into analogue music for your phones. The Asus card will convert your music so you can connect the RCA (the analogue output) to any amp you want.

The m-stage has no DAC so it would require you to connect to your Asus Card (via RCA), as Fallen Angel says.

However, the Aune has a DAC built in so you could plug this into the USB port on your computer and still get music out of your phones. The Aune has SPDIF too so you could use that instead of USB. This would bypass the DAC in your Asus Essense ST card.

So, you need to decide whether the card is good enough (in which case you only need an amp like the m-stage) or if you want a new DAC too (in which case you could buy the Aune or a seperate DAC and Amp).

I'm not going to recommend either option because I'm not familiar with your existing gear.

I can't answer that question. I haven't heard your sound card or the matrix (although it's apparently a heavily modded clone of the Lehmann BCL which I own).

However, my experience with upgrading is as follows:

I heard a difference between using my soundcard and the headphone out on my Marantz Int Amp (connected with 18th Inch Jack to RCA from the soundcard). The amp did a better job of driving the headphones and the highs and lows had more clarity.

However, I had a bottleneck because I was using my soundcard as a DAC so the audio had been deteriorated by the time it got to my amp. The clarity was just a result of the amp's ability to handle higher impedance cans.

I heard a bigger difference when I got a dedicated DAC because the amp was being fed with a good source.

Then when I upgraded my amp to a dedicated headphone amp, it was worth it and I heard a big difference.

You might want to buy something like a Matrix Mini-i instead of the M-stage. The difference is that the mini-i has a DAC and a Headphone amp so you'd not be relying on your soundcard for anything. Then you could keep the mini-i and buy a better headphone amp when funds allowed.

However, I'd research into amps (like m-stage) and DAC/Amps (like mini-i) before choosing. I can't recommend either because I haven't heard them. I'm just using the matrix brand as an example because you mentioned it earlier.

There are countless debates on this subject. If you search for the names of any DAC/Amps you find in the HeadGear section (button above thread), you'll find loads of people in the same position as you. Their experiences might help you to make a decision.