Not a lot to recommend for your budget range.Klipsch Promedia 2.1 are a good value, $85 for used ones (assuming you like a lot of bass from a sub-woofer)BestBuy has them new, on sale for $119.99. Dayton Audio B652 sell for around $60 for a pair, you will also need to buy an amplifier (Lepai $25-$30).
So your currently using a $500 headphone amplifier to drive $100 headphones? Guess it makes sense to swap the STX for an external DAC, Modi DAC being one option.Would you get a USB input DAC or optical input DAC?I would assume a USB DAC would be the safer choice for working with a new OS (Win 10).As optical would work off a sound card (on-board or add-on).
When you connect the FiiO E12 headphone amplifier to the FiiO X5, do you set the X5 to line-output (not Headphone output)?Do you max the volume control on the X5 and only control the volume using the knob on the E12?
Hopefully you disabled the motherboard's on-board audio, in the BIOS, before you installed the Essence STX.What headphones are you using?Switching from the Essence STX to an external DAC like the Modi 2 would help with any electrical noise that might be generated inside the computer case.Not sure it would make any improvement in audio quality.
The SB-Z's headphone jack has a 22-Ohm output impedance, not the greatest for use with headphones that are 38-Ohm (AD700X)But with the price the SB-Z costs for the possible amount of improvement in audio quality, it's still worth it Here is a used one on eBay.http://www.ebay.com/itm/181738265540?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT When you remove the Xtreme Audio card and it's Creative software, you might consider using a driver cleaner, before...
Get a (used) regular Titanium sound card ($40-$50) and a used FiiO E17 ($80?), off eBay.Plug the speakers into the Titanium's three speaker jacks and the E17 into the Titanium's optical port and plug the headphones into the E17The Titanium series of sound cards can send processed CMSS-3D headphone surround sound thru the optical port.
That is the XtremeAudio (SB1040) sound card, very bargain basement Creative card, replacing it with a newer Sound Blaster Z would improve the audio quality, plug the AD700X directly into the SB-Z.Hopefully you have disabled the motherboard's on-board audio, in the BIOS, before you added the Xtreme audio to the computer.
As your talking about CMSS-3D, I'm assuming your currently using a Creative Labs Titanium or Titanium-HD sound card (or other)?Then get the FiiO E11K headphone amplifier ($60) and connect the sound card's line and/or headphone output jack, to the line-input on the E111K, and the AD700X to the E11K headphone jack.The E11K less then 1-Ohm output impedance should be good for helping the 40-Ohm AD700X bring out it's best (like detail).