Pre/Pro with 2 MCH analog inputs plus..

Slightly off topic. Can somebody, please explain this? I have Bryston SP-1 Pre-Pro, which has one set of 5.1 inputs. My understanding is that Bryston's preamp section is 2 channel analog preamp "analogous to Bryston own B-25 stereo preamp". So, how does Bryston do 5.1 channel preamplification for multichannel analog source ( SACD or DVD-Audio)? Two front channels go through the analog pass-through and the rest through the digital circuitry of processor part?
The same applies to Audio Refinement Pre5, which is again a 2-channel analog preamp with "5.1 capability". Appreciate your clarification in advance.

Rros05666! So, what you are saying is that Bryston does have a discrete 5 channel analog preamplifier? That's were my confusion is coming from- I was under impression, that Bryston analog preamp is 2 channels only. The same goes for Audio Refinement Pre5, which according to Stereophile article by Kal Rubinson is "essentially 2 channel preamp" with 5.1 input. What I'm actually asking is this: Is there any way to make a stereo preamp capable of processing multichannel (5.1) analog input of SACD or DVD-Audio by simply (or not simply) employing 5.1 input?. Regards.