I've had the 300SEI and currently own the PX-25 in my system(Cain&Cain IM-Bens) and the two are totally different approaches to SET, sonically.
As pointed out, the Cary is ultra-lush, warm, rich sounding. Whether too much so or not is a matter of personal taste(way too much for me). An analogy might be that the PX-25 is fine silk and the 300SEI rich, blood red, velvet.
That's not to imply that the PX-25 is reserved or delicate sounding, just that it is more refined in every aspect while still delivering a musically vivid presentation. As one reviewer said "It's as if notes, instruments, are lit from within". It is also has amazing current drive capabilities for a SET, meaning some of the best bass response I've ever heard out of any SET amp. The PX-25 is truly one of the great SETs made.
As for the 300SEI, it lives for the midrange. A sort of hyper-reality in that band, but I can understand why loyal Cary fans love it. Unfortunately, I found it's low frequencies loose and poorly controlled.
I'm tired right now, but I'll try and be more descriptive tomorrow.