I have not owned the two products that you are looking at but do have a Meridian 568 upgraded to 568.2 for 4 years. In that time I have never had a software or mechanical problem with the unit. In reading the avsforum it appears that Theta CasI, CasII or CasIII all have software lockup problems to some degree, with the CasIII having the least problems. However, I do have two Theta products the Dreadnaught I and a Theta Carmen II, both of which are excellent products. If your are looking for high quality two channel sound then you would be much better served by buying a high quality tube preamp with home theater by pass. I have the Aesthetix Calypso and it is substantially better that the Meridian 568.2 and if your read the Audio Perfectnist they think any good quality tube or solid state preamp is better for stereo than the Theta CasIII. I have not heard the Meridian 861 but would be hard pressed to think that the Meridian would be better than a good preamp. However, If your interest is simply home theater then the Meridian or Theta is hard to beat. By the way if you go Theta, make sure it is balanced all the way as the Theta Amps do not like and unbalanced signal.
Good Luck.