You will get better two channel performance with the ARC Ref. 3 and the Cary SLP-05 than the Theta. However, you indicated (or perhaps somebody else did) that you are using the Dreadnaught amplifier. This will be your limiting factor with 2-channel performance. I have owned the Dreadnaught II, kept it for about a week due to its well below satisfactory performance in 2-channel operation. Sold it to a member in NY who wanted to replace his Halcro multi-channel amp with one that was reported to deliver better 2-channel performance. He kept the Dreadnaught II for less than a week and then sold it himself. He too indicated to me that it was weak with 2-channel performance. FWIW, my amp had three of the higher powered monoblock cards/amps and two stereo amps in it, so it shouldn't have been a power issue.
For absolute best two channel performance, you need either monoblock amps or a stereo amp. I have never heard a multi-channel amp do particularly well (I have owned several - Krell, Dreadnaught, Conrad Johnson, and others) with 2-channel performance. Of course everybody has different demands and considerations as to what is acceptable 2-channel performance.
For a 2-channel preamp, you have listed two very good and very different sounding preamps. The Cary can be purchased for much less used than the ARC. Another consideration could also be the Aesthetix Calypso and many others as well.
