My advice is using the highest gain with the lowest impedance output.
My personal taste is using treble tone +1 while left the bass tone shape in zero position, but ymmv of course.
Tyll at Innerfidelity has a review of the M8 which shows the frequency response curves of the bass and treble switches:
I tend to agree with his thoughts on the bass switch -- tends to make a resonant sound which I don't like. The treble switch, though, is great. I use it with my "classic" HD580 all the time. One notch up in treble is perfect.
I'm not surprised as the M8 sounds great with the tone shaping set flat so long as you are using a good headphone.
I also find that although the tone controls are subtle lifts, it's more or less like any EQ where it 's sort of dependent on the source music involved.
Sometimes it even varies track to track within the same album. So you can drive yourself crazy with constant EQ changes, or you can just settle on a given setting for any specific headphone and just leave it that way, which is what I've done.
There is nothing wrong with the flat setting and I think for a majority of good headphones it would be the preferred setting.
The bass switch just adds a very subtle lift, which imo is useless. If I want a bass boost, I will look elsewhere. The treble switch is much worse, can't discern any minute change. However, the overall sound is well balanced and both bass and treble switch is unnecessary.
Rudi did it. The only downfall he said, was it crushes the battery life of the ZX1. And you will need that special cable...
I have the same question for a Galaxy Note3 & HTC One; would i be able to connect using the iDevice input or only to the USB-B input? I'm about to place an order for the M8 and want to make sure i order the right cables, thanks.
I believe the answer for both the Galaxy Note, and the HTC One, as well as the Sony ZX1 mentioned in an earlier post is they will only connect via the USB-B port. You need an OTG cable which is different from a regular USB cable.
Only iOS devices made by Apple will run from the M8's iDevice input, which is set up to do the Apple authentication handshake for their own proprietary DAP protocol and no others. That also means the iDevice input is limited to 16/48, though CEntrance was said to be working on a firmware update that would allow some newer Apple devices to operate at up to 24/96 kHz.
The previous post mention of a ZX1 needing a special cable is correct, but it's not an iDevice cable, it's an OTG cable that allows the ZX1 or other hardware/firmware compatible Android devices to operate in USB Host Mode.
Here is a picture of an OTG cable in use with a Resonessence Labs Herus DAC, and an LG G2 Android phone:
You would need a similar OTG cable, you can even make one yourself by taking a standard USB cable and modifying it like this:
Better yet just buy one, there are some high-end/custom choices out there, or the StarTech pictured with the Herus and G2 is all of about $ 5.60 in the U.S.
When pin 4 and 5 are tied together, it signals to compatible devices to operate in Host Mode, this is also how you would connect a thumb drive, mouse, game controller etc to an Android device.