George - check the bottom of the review (I added a section).  I think the results will surprise you.

Thanks for that. They do surprise me. Now, I do not know what to believe exactly, and what to buy anymore so that I will like it. Probably the safest way is to always only buy what you know how it sounds and what you know you will like. 
 
I am still unsure if I should embark on X7 train, as it would cost me a lot and X5II already brings me much happiness. Also I thought to buy either Mojo, or to upgrade my over the ear headphones instead, as it would probably yield better improvements. 
 
Thanks a lot for being such a great help, and thanks for the time taken to do the graphs.

Tried different ears, seems like it's not me.
 
Just my other 'phones aren't as sensitive.
 
What a relief.

So after using the SPL meter to set the volume and with the blind tests, I still find that the high gain provides that extra dose fullness and dynamic sound. I am not trying to question your authority on this Brooko but the difference is like switching from iPod output to a real amp (not as big). I guess I am just trying to question why outside of YMMV.. Am I being crazy?

Not being crazy - although I really cannot see how it can happen.  If you like it this way, embrace it.  All I can tell you again (and the measurements show it - so it isn't me making it up) is raising the gain adds 4.9 dB of volume.  It also shifts the noise floor up by the same amount.  There is no other change.

Yea I'm writing my review for the X7 right now (writers block though) and it really did strike me that the supplied amp was kind of meh to my ears.

What earphones are you pairing it with?

If only the fiio X line made phone calls. 

Hi Brooko, 
 
Awesome review! No major details missed.
 
Let me ask you............ on a pair of headphones like the AKG K845BT (specs below), will the AM2 be loud enough? I mean LOUD, with no distortion? 
 
AUDIO SPECIFICATIONSDriverClosed-back headphonesFrequency Response20-20 kHzMaximum Input Power50 mWInput Impedance32 ohmsSensitivity102 dB SPL/VDriver sensitivity0 dB/1kHz 1mW
 
 
Also, wait so there's a Low and High gain on the AM2?? How do you switch it to High? Is there a physical switch or do you that via X7's settings??
 
Thanks so much!!!

@RockStar2005 - hard to answer some of it as it depends on how the AKG handles amplification.  Usually Bluetooth headphones have their own inbuilt amplifier - so that will be handling the "loud enough" question.  As long as the signal from the source is getting through sufficiently strong enough (and on the X7 - the Bluetooth seems to be pretty good), then you shouldn't have any issues.  Because you're essentially transmitting the signal, the amplifier attached to the X7 should have no bearing on volume.
 
And you switch the gain on X7 via the internal settings

Brooko, 
 
Wow..........that is MOST helpful! I just ordered the X7 (comes in tomorrow) thinking I needed to order the AM2 as well for BT listening on the K845BT. So ok, my headphones handle the amplification. So why is it then when I listened to music on the ZX100 Walkman and the less impeded/more powerful Astell & Kern Jr. that the Jr. sounded louder via BT on my 845 on the same songs? 
 
That's good to know that the BT on the X7 is pretty good. I'm sure aptX helps with that. 
 
Ahh ok. Good to know. I assume the AM1 that it comes with also has that Gain select option too?? 

Brooko, 
 
Nevermind. Maybe it just played louder using the Jr. I did some research elsewhere on Head-Fi, and someone explained to me that since the signal being sent to the headphones is digital, there is no need for the premium DAC to be involved. So the quality is the same as if I was streaming from my smartphone. I knew that stuff regarding the DAC for wired, but didn't know it applied to wireless too. 
 
Someone should really make a DADC chip.......... so that for BT listening, the BT headphones or speaker still get an improved signal. It would make BT MUCHH more appealing. Can't believe no one's done that yet. 

The thing is with Bluetooth, if you're using APTX it can transmit already at up to CD quality, and for the iOS people, A2DP transfers aac256 natively with no further compression.  So realistically the quality is already good enough.  If you wnat to see how good it can be - check out the XTZ Headphone Divine.  Amazingly good headphone.

Brooko, 
 
Yeah aptX is the best one out there now, 2nd only to Sony's LDAC, which ONLY works if both the player and headphones/speakers are LDAC-compatible. 
 
It's transmitting it, but if there's no "magical intervention" from the DAC, then to me it's not worth it to even bother with BT. I'll look into the Divine headphones too. Thanks. 
 
Just got this e-mail from FiiO after writing to them about when AM2 will be available............. FYI: 
 
Thank you for your mail and support to FiiO.

The interchangeable amp modules(AM2 will come first) for the X7 may be available in April if everything goes well. You can contact our authorized agents(http://fiio.net/en/stores) at that time.

If you have any other question, please feel free to contact us.
Here is our Facebook (https://www.facebook.com/FiiOAUDIO/), and you can "Like" it to know more our new products, preferential activity information and so on.
Have a nice day! 

Best Regards,
FiiO Customer Service

Brooko,
 
So the XTZ has a DAP in it. Is that like a DAC? I mean, how good is the DAP though? The price is good though ($179), but I wonder if it could compete with like the $300 Oppo HA-2 amp/DAC? 
 
http://www.xtzsound.com/product/dsp-headphones

Perhaps better to leave this comments section for discussion of the AM2.  I have a review posted of the Divine - http://www.head-fi.org/products/xtz-headphone-divine/reviews/15349 - and there is a thread for discussion of the headphone - http://www.head-fi.org/t/795113/xtz-headphone-divine-discussion-thread

Ok cool

Hi Brooko, any comments on AM2 vs K5 ? Interested in buying one or the other. Which has the better sonics and amplification power to drive headphones up to like a Sennheiser HD800. Also which has the larger 3D sound stage and details retrieval ? Thanks

Great information - thanks!

Great review thanks. I may wait on the AM3 and 5 release. We all hear differently which is to say subjectively. I find that the AM1 has a difficult time driving my HD600's, but paired with the E17k Alpen 2 all is well. Really brings the HD600's to life. I've been impressed thus far with the X7, so I am hoping the AM3 and/or 5 will be able to power the HD600's so I can use the E17K strictly as a dac. Thanks again.