Geoff, first lets discuss two channel source material, since your understanding may not be entirely correct. Depending on what you mean by "sound fields", the result may or may not be "fictitious". If the reference is to fields such as Jazz Club, Orchestra, Beer Garden, Sportin' House, etc., these add artificial echo and other effects to the content and the result may fairly be termed fictitious. If, however, surround processing such as DPLII or Neo:6 is the subject, this takes surround material which is naturally present and was mixed into the front channels(there was no place else to put it), extracts it, and sends it to the surrounds where it belongs, making the result a little more realistic. I always use a surround processing mode, usually DPLII, with two channel music material.
With 5.1 channel material(DVD, Blu-ray, or whatever)you shouldn't set "Direct" since that plays the material exactly as it comes, and although you'd be getting 5.1, it would be without bass management that would send bass from the five speaker channels(set "Small") to the sub. The only bass the sub would receive would be the low frequency effects in the .1 channel that are occasionally present during movies. One of the DD or DTS modes should be selected. If your speaker setup is 7.1(you didn't mention it), you can use DPLIIx to expand the surround content in the DD or DTS side channels so that the material which should be imaged in the rear is sent to the back surround speakers.
