I see a lot of inquiries about how to use headphones with the AX8. I had that same question when I got my unit back in April, 2016. I read the complete thread about needing a headphone amp or using the headphone jack from a recording interface. Since I don't have a recording interface (nor do I need one), I thought it might be helpful to some to share my set up for using my AX8 with headphones.
I purchased a dedicated headphone amp for this purpose. I need to use my AX8 with headphones to learn songs for my cover band, to practice quietly at night, and to do preliminary programming of presets and scenes (final adjustments are made at gig volume with an FRFR powered speaker). This set up lets me mix the volume nicely between my iMac iTunes app and the AX8.
The first photo shows the connections to the back of the headphone amp. From left to right is 1) the output coming FROM my iMac's 3.5mm stereo headphone jack, with a split to RCA RIGHT and LEFT. Using RCA to 1/4 mono adapters, I plug each into the respective R and L inputs on the back of the amp. Of course you can also use 3.5 mm stereo cable split to 1/4" R and L mono plugs. (I just happened to have the 3.5 mm to RCA cable on hand). 2) To the right of that is the jack where I plug in my headphones. 3) At far right is the power adapter.
The middle photo shows the back of the amp with everything plugged in. The headphones can be plugged into any one of the four outputs.
The right photo shows the front of the headphone amp. As you can see, it fits nicely under my 27" iMac monitor. The 1/4" plug in the foreground comes from the AX8. I run a FEMALE XLR from OUT 1 (LEFT) on the AX8 to 1/4" MALE TRS which plugs into the STEREO INPUT jack at far left on the front of the amp. The MASTER headphone volume is controlled by Volume 1 on the front of the amp. I control the level of the music coming in, by using the volume control in iTunes, and the AX8 level with the OUT 1 level control. I'm running the AX8 as MONO out (Sum L / R, which I do in a live setting anyway), so I hear the signal from both sides of the headphones, (not in true stereo of course.)
This particular amp is often on sale at Amazon.com for $39.99 It sounds very good, it's quiet, and it won't break the bank. goo.gl/Hzw8lL