Its just that naim is not very good at that type of price, but it's my opinion. If you brought your pmc's to mine (or could do so) I'd show you what I mean Mark.You'd see how much more you'd hear in detail retrieval and dynamics which is hugely important in music, probably the most important. pmc twenty are smooth already and they need a kick in the teeth of some detail imho. Otherwise way too laid back. I'm not talking in your face treble but just nice balanced treble with bass.
If I were the original poster I'd look at speakers to improve things, if he isn't satisfied, and I'm qualified to comment as I used to own tannoy dc6t se's. I think he likes a smoother presentation than tannoy remembering a previous post, so keeping the audiolab stuff and changing speakers at the same or similar price of the atom (£1750) would yield more an improvement than the atom and get the balance just right. A speaker that is smoother is needed. I'd think the atom would change the presentation too much in the opposite direction for those tannoys and it might not be to preference, and will make the tannoys more bland beyond hisn tastes, where they need a bit of sparkle to perform really well.
Thats essentially what you are trying to achieve as you upgrade, get things like dynamics and detail and bass responsiveness, which is basically what the really best hi fis do that are up there in the £25k plus range like when you put pmcs like 25-26 or fact 8/12 with really top naim stuff. There is a concern for me too, when you have a brand like naim producing really quality stuff at the top of the range, the lesser stuff doesn't see much trickle down from the top stuff, so it's like the designers have given up a bit or know they are seriously compromising. With a brand that is better value like audiolab or Cyrus etc, with prices between the high specced stuff and lower specked stuff more similar, you are getting the best efforts of the ideology and design of the designers, which I think means better sound quality at the price. It's a bit like buying a Volkswagen up! You can buy a Ford Fiesta which drives a lot better, if analagous to sound quality we agree driving pleasure is what the car is mainly for. But the up doesn't drive as well, doesn't have the same features and is more expensive. It's the naim versus the Cyrus or audiolab.
The tannoys are a £1000 ish speaker and going up near to £2000 like the atom price, but in speakers, would get the best returns if sq is the aim here. Especially trading in the tannoys.