So buy a 990 and be done with it.

But, the 990 does not have Logic 7. DPL IIx and Logic 7 have the same end goal in mind: 7 channels of output info from 2 to 5 channels of input info. They are similar, but not the same. There will be audible differences.

Also, because the 990 digitizes the 5.1 inputs, there can possibly be differences there as well vs a pre/pro with a straight through analog path.

Plus, some people consider more things than just audio quality when making a buying decision like this. Build quality, ability to do hardware upgrades, where the unit is manufactured, etc.

If it was as simple as you put forth, then all of us would already have a Rotel 1068 (which had DPL IIx long before the 990) and life wouldn't be interesting at all.