I appreciate all your information, which has encouraged me to place my name on the notification list for the (currently unavailable) DD-5.1 preamp/processor.
I've been able to locate the Dolby website
http://www.dolby.com/
which has been helpful in explaining the distinctions between the various surround formats that Dolby has developed. From the descriptions given on the site, it seems that the basic Dolby ProLogic decoding is very similar to the "passive" Dynaco system I mentioned earlier, because they both route L + R signal information to the front center channel (the Dolby site states specifically that the ProLogic circuitry attenuates this center signal by 3 dbl, since the combined signal would otherwise be too loud) and both the Dynaco and Dolby systems route a L - R differential signal to the surround channels, (through a filter that blocks frequencies below 100 Hz and above 7000 Hz to avoid confusing localization effects, though the Dynaco decoder does allow the listener the option of turning the filter on or off.) Dolby Prologic also adds a slight time-delay to the surround signal to further simulate sound that has reflected from the surfaces of the performance venue itself; the Dynaco decoder does not include this feature.
My one concern about Dobly's ProLogic decoding is the bandlimiting filter applied to the surround channels. I have bought four identical, full-range, excellent quality, "active" loudspeakers capable of reproducing music truthfully from 30 Hz to above 20,000 Hz, with the desire to benefit from their fidelity in all channels, front and surround, because I knew that my primary purpose for these audio components would be Classical Music listening (rather than TV viewing or gaming -- believe it or not, I don't yet own a DVD player, a television, or a desktop computer, though these are all gadgets I'm considering for eventual, future purchase.) I wonder if I wouldn't defeat the purpose of having acquired these fine loudspeakers by adding in a decoder like the Klipsch DD-5.1, whose Dolby ProLogic circuitry would eliminate the highs and lows the speakers can faithfully provide. (I'm a fan of Baroque organ music and, now that I've found speakers that can do justice to the lowest pedal-notes and the highest pipe-overtones -- essentially free of any distortion -- I'm a little reluctant to eliminate these portions of the audio spectrum!) I should probably also mention that all four of these active speakers have individual high frequency, low frequency and volume controls, so, I can already tailor their response to the characteristics of each particular recording, when needed, without truncating large segments of their full sonic range. Does the Klipsh DD-5.1 allow you to turn off the ProLogic bandlimiting filter, if you wish? If not, I might just find myself using only the "venue modes" on the device, or employing it fundamentally as a pre-amplifier (where, I've assumed, it would function quite well) between a SONY DVD / SACD player I'm considering to buy (which includes its own Dolby Digital/DTS 5.1 decoder) and the power amplifiers of my present speakers. I've honestly been enjoying this equipment "research project" I've embarked on, trying to learn as much as I can about these surround processors, and I'm grateful for all the "educational answers" I can find. Best regards...