Convict, I used the same Vishay film caps in my pre-amp and have the same sonic findings as yours.
In the end, I replaced them with WIMA MKP series, yeah! the red ones! I found the Wima caps to be more transparent overall. But if you do have an ear piercing experience, then the Vishay caps would tone the harshness down, at the expense of some transparency.
I would even ask you to go as far as changing the whole Bennic cap to Mundorf ones for the full effect!
