ALL plugins that use new style preset work in XTC... I use them all the time...
Including Modular2/3 shell... But you CAN'T alter the patch itself once loaded inXTC...
STSx000 are exception, they don't even load...
Adern KickMe was the first one i tried really, the minute it was out, it was working perfectly in XTC for me
All SpaceF effects (the excellent Echo2/2i/2m/3 all works), the newer celmo devices like X-Machine, Vintage Pro Series tape Delay, Ganja delay, Sonic Attack, D-Tune, Synthetic, sf-PulsedFMSynth, D-Comp, Master-D, D-Filter, and many more...
I once posted somewhere a screenshot of Ken's 73' modular patch running in XTC mode in nuendo
XTC is great, but lack commitment from cwa and developers. A couple of small utility xtc devices could complete all that is needed in XTC and really shine compared to other solutions.
Continue your experiences, and mybe that'll interest some more people to xtc

Who knows, maybe it'll be the next 'thing' into audio land
I know i'm sold allready. Just wished my system was more stable (pulsar1)
