Haven't done the detailed troubleshooting that some of you have, but this is what I am hearing.
DD 5.1:
L / C / R + L Side and R Side = Balanced to amp
One RCA pre-out ---> splitter ---> two powered subs
No pops – everything worked perfectly
Added second amp and two Rear speakers…
DD 7.1:
L / C / R + L Side and R Side = Balanced to amp (same)
One RCA pre-out ---> splitter ---> two powered subs (same)
L Rear and R Rear RCA outputs on 990 ---> 2-channel amp
Pops are heard ONLY from the Rear speakers when changing satellite channels (DISH 622), or simply pausing and resuming the playback.
Will take a closer look at DVD playback and see if there is also popping that may be related to acquiring a DD signal as mentioned above.
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Outlaw 990 / Cinepro 3K6 MkIII Amp (375 Watts X 6 - balanced) / three M&K S-150's (LCR) / two M&K SS-150 surrounds / two M&K MX-350 Subs (all THX) / Clark Transducer / DISH 622 HD Sat. Rec. / PS3 / Panasonic PT-AE4000 Front Proj. / 110” Stewart ST 130 Screen