I have a decent 2 channel system consisting of the following:

Sony 222ES SACD MC CD Player
HK 3475 (110W/chnl receiver)
Bohlender Graebener 520 DX Ribbons
Parts Express powered Titanic sub

My HT is in another room, this system is for music only.

Anyway, I heard a SACD MC demo the other day and I am hooked! Gotta have it. I started looking for a pre or receiver that will allow me to listen to both 2 channel stereo and MC. Here are my questions:

1. I intend to buy 2 good surround speakers, do I really need a center channel speaker too?
2. I am using a powered sub so I don't want to use any bass management. I want the sub on at all times for both 2 channel and Multichannel. I would like to continue to power it from the main audio outputs (speaker level) as the amp sums the channels to mono. Can I do this? I don't want the sub going off when I play 2 channel (most of the time).
3. My HK has pre/amp inputs and outputs so I would like to use the more powerful HK amp to power my BG ribbons. Any problems?
4. Can I use the analog inputs to handle my Sony MC SACD player?

Looking to maintain the best quality 2 channel sound while adding multi-channel capability. Anything else that I need to worry about?

Thanks a lot for your help!!