I have my digital cable box hooked up the the 1070 using the analog inputs. When I put the selected input in Bypass mode I get a nice 2ch stereo signal that has plenty of volume and I don't need to turn the volume dial up much on the 1070. However, if I put that same input into any Dolby mode the volume produced by the amp drops way down. I need to crank the dial up by at least 15dB to get it back up to the same volume it was producing in Bypass mode.

Does it really take that much more power to produce the Dolby signal as apposed to Bypass?

Also, I don't have any surround back speakers and have them set to "none" in the config. Am I supposed to still get the PLIIx options?

The signal path is very different in Bypass and in other modes. The signal is being converted from analog to digital, processed by the DSP, divided into more channels, and then converted back to analog. It's not really a case of surround processing modes requiring more power as it is a result of the differing signal path.

One thing that I wonder about... Did you calibrate your audio levels for each channel? Not doing so (or being off in the calibration) wouldn't yield a 15dB difference in level between stereo and surround, but if the center and surrounds were a bit lower there could be a few dB of difference.

While I'm at it - is there a reason you aren't using the digital output from the cable box. Cable boxes aren't known for their quality DAC's, so unless you have one of those cable boxes that doesn't always provide a digital output (some of the older Scientific Atlanta boxes are this way, and don't provide digital output from some analog cable channels) I'd recommend using the digital output if one is available.
Without back surrounds, Pro Logic IIx and Pro Logic II are identical. PLIIx only needs to be identified separately once you have a 6.1 or 7.1 setup.

I use a digital coaxial cable in the Digital Audio out on my cable box and I do not have the problem you are having.

Yup. The inputs are configured for different levels. On this example the input is staying the same, I'm just changing between Bypass and Dolby. I understand what you're saying about having to do all that processing and how it can have an affect on the signal.

Thanks for the clarification the PLIIx. I was thinking that they wouldn't even show up if I didn't have the back surrounds configured. At least I understand now what's going on.

And ya, I need to try the digital out. I had it hooked up once but with a cruddy cable. I need to move my good digital coax off of the dvd player and try it off of the cable box.

Thanks for your help guys. Still getting used to the new amp and getting it all set up correctly. Really appreciate the wisdom and help!

Does this mean that the settings in the 990's CHANNEL CALIBRATE menu have been adjusted (either using the auto setup or manual setup via an SPL meter)? Just double-checking...

Yes. I've calibrated them. Not to the exact perfection of using an SPL, but it should be close. I need to invest in an SPL.