Ben,
CAST technology was developed by Krell and used exclusively by Krell. Krell has an excellent explanation of cast on their website : www,krellonline.com/cast.html
My understanding of CAST: all electronic audio equipment operates in current mode. The electronic current is converted to voltage, transferred by cable to the next piece of equipment where it is transferred back into current, modified as necessary, converted back to voltage, transferred by cable to the speakers. Everytime the signal converts from current to voltage and back to current some distortion/noise can occur. Krell CAST equipment uses an extremely high frequency bandwidth (>2 MHz) and transfers the signal in current not voltage. This eliminates distortion that can occur durring the conversion from current to voltage (and voltage to current). Also, Krell claims that since the signal is transferred along their special CAST cable as current rather than voltage, there is no signal degradation effect produced by the cable. They claim, with CAST, the signal remains unaltered from source to speaker. I believe the Krell amps ending with C (400C, 600C, etc) were the first generation Krell amps to use cast. They have CAST in their high end amps, processors, pre-amps, CD/SACD players. I believe the Krell KCT was the first pre=amp with CAST.
CAST is very interesting, innovative technology. It is a great idea. Does it work? I have never A-B'd to compare. Reviews state that it does make a significant improvement. My guess is that it probably does make an improvement. No one else has CAST, Krell owns it.
The S275 does not have CAST.
I don't know anything about the PS Audio DAC/pre-amp. Do you like it? It probably will be fine with the Krell S275.
The Krell 150 mono blocks probably sound very similar if not identical to the S275. But the S275 will give you more power. The choice probably depends on the speakers you will be driving.
Once you get into the 450MCX, Krell KCT level of equipment, things like cable (Krell CAST cable, etc) become important considerations, also, more expense.
SFSTEREO:
The pre-amp section for stereo music in the Krell Evo 707 is fantastic; far superior to the stand alone Krell KCT pre-amp I owned previosly. If you are able to audition one for music, make sure to go into the menu and sslect pre-amp for the CD player and/or record player. This bi-passes the digital processor. The Krell Evo 707 is an amzing piece of equipment. The sound is extraordinary for home theater and stereo music. I traded my Krell KCT and Lexicon MC12B when I bought the Evo 707. I cannot say enough about the Evo 707, you have to check it out.
Good luck.
