You may want to sacrifice some portability and opt for a more refined amp for your sibilance issue as neither the ak120s nor 1p2 should give you too much problem in that. I no longer have the pico slim and didn't had 1p2 when I had so I am not sure how well they paired. Check out the mass kobo 385, excellent sounding pair with 1p2.
901 with upgraded amp board is also a good choice and seems like a deal for its price comparatively.
Its not the amp- I've also been using my alo international and still get sibilance with certain vocals; I've also tried just my rwak100 and 1plus2s and get sibilance too. I think I'm just particularly sensitive to it