There simply isn't a single stand-alone sampler that has enough real-time and modulation flexibility, so most people use modular patcher programs (Reaktor, Max/MSP etc.) for that kind of stuff and more.

I would recommend either Usine or Bidule, because they are the easiest ones to use and can be used as VSTs in addition to running as stand-alone applications. And they can host 3rd party VSTs in patches, so you can throw in e.g. Alchemy and just attach controls to it + add effects and more: 

http://www.sensomusic.com/usine/

http://www.plogue.com/products/bidule/

Another option, although a total overkill for just simple sampling, is [Kyma][1].


  [1]: http://www.symbolicsound.com/cgi-bin/bin/view/Company/WebHome