After reading this in your previous thread,
My goals:I suggest you search threads to see how many have struggled with pre-pros, some even mega buck, and they were never satisfied with the 2 channel performance compared to dedicated 2 channel. Using a HT by pass/pass thru, as you are currently doing, gives you the best scenario when 2 channel is important.
2 channel (no subwoofer) listening at the same or higher quality than the current 8004-Cambridge chain. I should say that I am VERY HAPPY with my current 2-channel setup.
Higher quality HT audio, either from the improved processing of a prepro, or of the Oppo.
Get my feet wet in multichannel audio. Currently my SACD listening is done in stereo only.
If you want to improve 2 channel, upgrade to a better integrated amp with HT pass thru, or 2 channel preamp/amp with the preamp having HT pass thru.
If you want to improve HT, I see your biggest problem being mismatched speakers. In the very early days of surround sound with only Dolby Pro Logic, the rear channels were mono, limited bandwidth, and even less power than the front in the A/V receivers. In that case I would say rear channel speakers might not be as critical as the front. Today is a totally different story since 5.1 surround sound is 5 discrete full bandwidth channels with a separate subwoofer channel. Take a look at the picture here,
Obviously, that jet would sound the same, front or rear in the room, with another pair of B&W CM7's for the rear, and some people would do just that. The best multichannel system is all identical speakers. The next best thing is to use timbre matched speakers. As you keep dropping quality and/or moving to another brand, then that 747 jet on the front may sound like a 2 passenger prop aircraft on the rear. Also, think about it moving from left to right across the front when the center is being used. Matching the rears and center with the fronts will give you a better balance to the sound throughout the room for movies and multichannel SACD. Keep in mind that the automatic setup in A/V equipment, regardless of what type it is, only adjust for room acoustics, and not for the difference in sound character between a JBL and B&W. It's not OK to use different left and right speakers, so why should it be OK to use different front and rear speakers.