Comparisons to all located here, but the AM5 was my choice as you will see in the review.
Also updated my X7 review to reflect: http://www.head-fi.org/products/fiio-x7/reviews/14715
PLEASE NOTE: The biggest win overall was a huge increase in SQ from the original outstanding sound through firmware updates. To reviewers that have only heard the original tour model, it is worth a second listen.
A big thank you to FiiO for allowing me to participate in the tour and try out the amp modules. I am so glad I signed up because it has really changed my opinion of the X7 and has really invigorated my love for the DAP. Here are some brief impressions after trying them all out.
Brief Initial Impressions
AM2 Amplifier Module
Warmer and Somewhat Darker sound
Not as detailed, smoother sounding
Treble feels less extended, not as airy or neutral sounding (which affects the soundstage a bit)
Thicker midrange that is also quite lush, treble takes a bit of a back seat
Stronger mid bass - almost an L shaped presentation, bass has more body and definition and not bloomy
Perfect pairing for Brighter gears
Not an improvement on AM1 but rather a different flavour
AM3 Amplifier Module (SE , Balanced to be tested later) aka what the AM1 should have been
Linear and more detailed with beautifully extended treble
Holographic and large soundstage that also sounds effortless and natural - thanks in part to how jaw droppingly good the treble is
More detailed and musical at the same time, everything sounds more alive and dynamic (not to be confused with forward treble)
Detailed and engaging midrange that is also thick with detail and emotion and ambience - perfectly balanced with the rest of the range
Tighter bass with improved resolution and texture (really noticeable with wind instrument pieces and standing basses)
The entire frequency range is immaculately balanced and if you could have only one module IMO this is the one to get.
Sounds great with ALL gears and all genres; it sounds so good that I really had to force myself to take off the AM3 to try the others and after testing the others thoroughly I promptly went back to listening to the AM3
An improvement over the AM1 in every possible way except battery life
I havent had a chance to test out the balanced output yet
AM5 Amplifier Module aka having a car battery in your pocket
Most powerful of the bunch, quite a bit of driving power even on low volume, relatively speaking
Slight warmer sounding than AM1 but not as warm sounding as the AM2 - strikes a fine balance
Retains the detailed and dynamic sound signature of the AM1 while smoothing out the treble a smidge
Still sounding detailed and energetic (just not as energetic as AM1), everything sounds more alive and dynamic
Higher resolution than Am1 - so better separation layering and positioning
Treble feels a tad softer but still beautifully detailed and sparkly - well done imo
Unlike the AM2, the AM5 soundstage doesn't take a hit compared to the AM1, the soundstage is once again holographic and large while one again sounding effortless and natural
Detailed and engaging midrange thick with detail, emotion and ambience (a more lush AM3)
Bass has more quality and quantity - bass is more forward and separated from the mix - easier to delineate
Bass sounds more detailed with greater impact and slam - VERY tastefully done that doesn't ruin tonal balance
Again sounds great with ALL gears and all genres - makes my HD600 sing!
This module easily powered my HD600 while I worked around the house the last few evenings - 'nuff said
In summary the AM5 module has all the things that made the AM3 special (just not as airy sounding) but with better bass, a sweeter midrange and more power
These impressions are all based on comparisons against the AM1 only unless otherwise stated
I am legitimately blown away how much better the X7 sounds better with the other amp modules as compared with the AM1. While I have always enjoyed using my X7, I never really loved it, that is until I listened with the AM3. The AM3 SE output is what the AM1 should have been, something truly special that has me staying up late nights listening.
In fact I have purchased more music over the last few days than I have over the last few years (I am mainly a Spotify user) to listen with the X7 + AM3 combo. Yah, its that good. The X7 is officially a TOTL DAP in my eyes and for all the people who has only heard the X7 + AM1 combo, PLEASE do yourselves a favour and give it another try with either the AM3 or AM5 (or even AM2 if you found the AM1 too lean).
If anyone was curious, my preferred set ups at the moment:
FiiO X7 + AM3 (SE Output) ==> Sennheiser HD380 - this combo has me losing a lot of sleep but man does it sound so good!
FiiO X7 + AM5 ==> Sennheiser HD600 & Beyerdynamic DT150 (Still a great sounding pair, best for when I am sitting down working)
If anyone has any questions please don't hesitate to ask, I will try my best to answer them.
Here are some pictures that I stole borrowed from the FiiO site that explain the modules in detail