Sorry I went offline missing this last night.
I've tried both ways and to be honest there's not much difference. I tried setting an E11 to 6/8 and using the source which certainly helps limit distortion however because I like using the E11 volume knob I usually turn the source up about 3/4 or for higher impedance you can go a little higher. At the moment because I use stock sansa firmware I turn the volume all the way up on the clip+ then back of five clicks downwards. You can actually test these limits.
What you do is set your amp to full volume. then bring the source up click by click until you hear distortion, when you hear it back off two clicks, leave it at that setting. That way you now have an amp that can run at full volume. Of course you can give lee way a little but your ears will tell you what's right.