So far, I've tried a number of different combinations with the B3:
Geek Out 1000
Fiio X3 (unamped)
Recently, I've tried the the Fiio X3 and DX50 lineout to the Pico Slim amp (I couldn't try with the AK100 since I've sold it, and there isn't a lineout function afaik), and the results are very favorable, especially for the X3.
The sound is similar to the HUD-MX1, essentially colorless. However, the Pico Slim definitely brings out the bass more. Bass is tighter, hence by extension appearing more textured, and also hits harder. B3 users who wish for more bass without bloat (quite a few from what I remember) will find this a welcome option.
The Pico Slim also has an in-your-face presentation, while soundstage, in particular depth, is increased as well. The result is... I don't know how to explain in words because I'm not sure what it actually encompasses from a technical standpoint. Fiio X3, and by nature all Fiio products, are slightly constricting sounding due to it's mid-centric ness. This is nicely 'counteracted' by the addition of the Pico Slim to the chain. Particularly for the B3, it's greatest sonic trait is it's legendary, enveloping 3D soundstage, and the last thing any amp should do is to compromise on this. The Pico Slim actually improves on this all-round effect, and the difference is not just subtle. It's somewhat like moving from in the middle of a nice concert hall, to row A of a large expansive stadium with state of the art acoustics. I mentioned before that the AK100 with the B3 gave me an expansive soundstage like no other combination before. In comparison, I would say that the Pico Slim has a very slightly less wide soundstage, although instruments are more pulled towards the sides.
In terms of clarity, detail, separation, the AK100 was outmatched by the GO1000, and now equally by the Pico Slim as well. I would however still say that the GO1000 is a little more dynamic sounding than the Pico Slim. B3 is naturally a flat sounding IEM which appeals to 'purists'. The Pico Slim certainly provides the B3 with greater oomph for more universal enjoyment.
I do not detect significant changes in imaging, nor in the treble frequency. I would note that the B3 has a tendency to become a little grainy in the highs especially with multi layered instrumental music. I'm not too sure if if I should isolate it as a result of insufficient amping, poor MQ of the source file, or simply a limitation of the B3, but I'm pleased to say that I threw one of the most tricky tracks I have at the Pico Slim addition (Foo Fighter's 'Walk'), and most (but not all) of this harshness was smoothened out. That was a hugely satisfying outcome for me.
Overall, where would I rank the X3 + Pico Slim combination with the B3? I would probably say that the Geek Out 1000 is still the better sounding pairing overall. The Pico Slim has the better (best so far!) soundstage, but the GO1000 edges out in it's blacker background, lower noise floor, and more 'engaging' sound (though the Pico Slim is already darn good as well). When purely considering synergy with the B3, the GO1000 still does more to distinctly augment the sound of the B3 positively.
If I were to have an overall preference ranking of all the combinations so far on strict personal opinion and preference:
GO1000 >= X3 + Pico Slim > AK100 > MX1 > E07K