I've never heard the power amp you're using (not common in Australia unfortunately), but I do own a P8 and I am familiar with the sound of the NADs.
I would expect that the first thing you'll notice is that the sound seems less bassy and colder (more forward, without being actually forward). Then you should notice that the base is actually still there as strong as ever (if not stronger and richer), but lower mid and upper bass is less emphasised and this is what makes it seem less bassy. This is what neutral sounds like (NAD are intentionally voiced warm). You will notice more detail and sonic texture within notes over the entire audio range, and you'll note you've swapped artificial warmth for accuracy and clarity. You'll notice an increase in attack speed (if your Omegastar has the jones to keep up, which it reputadly should have), and also in decay time; intramusical silences will stand out, and be dead silent. Your Omegastar will be able to shine and it'll sound like it has a slight extension if range at both top and bottom. High notes will be sweeter, and tricky to reproduce things like violin solos will be smooth and heart breakingly sweet. THings like cymbals will have more 'shimmer' but without the extended wiz of artificial treble extension (unless you have speakers that wiz anyway). You should find mids to upper base reproduced with far more clarity. The speed of the pre should allow the omegastar to show its transient authority too.
At first you're going to wonder if you made the right decision, then you'll get used to the sonic neutrality over a few weeks, and if you plug your NAD back in, you'll find it murky and quite lacking in detail by comparison. At first you'll miss the induced warmth (you may even long for a tone control or an eq), then eventually you'll be over it and you'll never want for it again.
Of course, I could be wrong.