I just listened to Diana Krall > Sansa Fuze > various portable amps > PK1 with stock punched foams. Amps were:
- TTVJ Portable Millet
- Meier Move
- XIN Supermicro IV 4/11
- FIIO E5
Sibillance was well presented with either amp, while Millett was least sibilant. Anyway, I doubt that you will sucesfully cure the problem with an amp. You may try alternative ways, like using higher bitrate or trying another foams.
I beleive that the best solution is an upgrade to OK1, which excels on vocals. See the quote from my OK1 vs PK1 comparison:Mids quality: +1 to OK1. OK1 mids are more neutral and are very distinctive - I hear every singer rather than choir in a whole. PK1 mids are warmer and less detailed. PK1 mids are more forward also, closer to you.
Highs quality: +0,5 to OK1. Update 1: At first version of the review I gave +1 to PK1, later I carefully listened to drum solo test track and revised the score. Highs of PK1 are stronger, OK1 highs are softer and more detailed. Due to more agressive highs, PK1 sound more lively with fast rock, while OK1 sounds a bit boring. On the other hand PK1 may produce some sibilance on vocals, which OK1 never does.
Start with a link in my sig, if you are interested.