i always used to crank up the gain when trying to get that singing sustaining sound, that seemed to be the only way to get it. i've found now that reducing the pre-amp gain and pushing up the master is much closer to that type of sound. basically it's much cleaner than you think, the power and sustain come from the power-amp working hard rather than pre-amp distortion. i know Gary Moore is known for using a Marshall Guv'nor overdrive ( which i have) and i'm pretty sure Slash has used overdrives, but i remember that he actually used an EQ pedal to boost his mids and overall level for solos. i now use a treble booster pedal rather than an overdrive.
i reckon you should roll back the gain, maybe even use the cleaner channel if it does a crunchy sound, and then use your pedal with the level turned up to hit the amp harder, hopefully driving it into a creamy sustain.
of course the problem is volume, at low volume you may just have to stick with loads of distortion