Yep, caphill is correct in that most processors will have a true "analog passthrough" mode. It's correct that it will not do bass management/crossover, since you are just sending 2-channel analog directly through to the left/right speakers only. Even Krell has a mode like this.
For HT digital sound quality, caphill may be correct. The differences in sound quality for 2-channel analog passthrough introduce a new segment: the analog input stage. This can actually make or break the sound quality of an HT processor when used as a 2-channel analog preamp. If caphill is saying that Sigma is better for 2-channel analog, then Classe may have built a better analog input stage. The SSP-800 was really intended for HT processing, so the analog inputs stages may not be as good.
I do know that the Bryston SP3 uses another set of their discrete Class A circuits for all 8 channels of the input stages, so caphill's comment about the SP3 being great for 2-channel analog audio could be correct. I still don't like the SP3 architecture - where they have a big main power supply, but no localized power supply capacitors around the actual analog circuits. I have found through R&D that this lack of localized power supply will give you softer mids/highs and a lacking in attack/resolution. To each their own, though. One thing I like about Krell Class A circuits is that they have a lot of localized caps (typically soemting like 6 x 47uf capacitors around a single channel audio stage). This gives them extreme amounts of attack and resolution. However, the Krell main power supply is typically undersized (especially in their HT processors) so that you get the typical Krell thin/bright sonic signature.