As said, the PURE DIRECT mode bypasses all bass management settings so all speakers would be regarded as LARGE and none of the receiver's crossovers would be in effect. THe only thing being conveyed to the sub while in the PURE DIRECT mode would be the LFE channel if and when present. Stereo sources do not include an LFE channel and only multichannel soundtracks include this.
You'd be advised not to engage the PURE DIRECT mode if you've sat or small speakers reliant upon the sub for bass. You'd ideally want floorstanders if wishing to benefit from using the PURE DIRECT mode.