Hello David,
yesterday and today again I've listened to your examples - I think there is nothing wrong. The "dreadful sore thumb" sound is coming presumably of beeing good in tune of the trombones (in this deep range). Sometimes this happens with real brass player too, when they are v e r y good in tune. (I have this "effect" on my accordeon by using chords in deep ranges too!) I remembered on the tune "Zorba the Greek" by Herb Alpert when he is playing the two trumpets (obviously overdubbed). Listen to 2:41 until the end. That should be the same "growl effect". (Sorry for this example far away of the style of your piece, but maybe it makes clear, what I'm talking about.)
Maybe you try out some "detuning" with VI Pro?
Hope that helps and hoping that's the subject of your issue?! My apologise if I'm wrong.
Best
Frank