OK - first thoughts from a couple of hours testing.....
Sonarworks has a profile for my headphones (or certainly very similar) - that seems quite effective. I sort of knew there was a bass boost on them, but the way Sonarworks reduces that sounds much more natural than if I try and use EQ to do it.
I tried experimenting with the 'simulate' options in Sonarworks - there's only a few options in there (I assume that's because it's a demo?) but there are options for AKG701 and Sennheiser HD650... I'm not sure how accurate they are but (to my ears) what they are suggesting from the AKG 701 is quite an 'alteration' to the sound compared to 'flat'. The HD650 seems much closer to 'flat' (to me) - but I guess that is why they cost more. (This ties in with what Soundwise posted earlier about frequency graphs - thanks again for that).
Couple of things about the plugin itself - doesn't seem to respond too well to touch screens (not sure why - most buttons seem to need two 'taps' when a mouse is a 'single click'). Also, the plugin drops the volume considerably - this is quite annoying when trying to A-B test this against other plugins.
Also tried Morphit - doesn't have a preset for my Sennheisers - so harder to test. After hearing the 'proper' correction on Sonarworks I managed to find one on Morphit that was similar for testing purposes. Seems to do a similar basic job (and is cheaper). Doesn't have the option for simulating other headphones / speakers, but a) is cheaper and b) doesn't appear to cause a volume reduction.
Lastly looked at TB Isone (speaker / studio simulation). Really not sure about this.... Firstly, it is another one with a significant volume reduction - and to make this worse there doesn't appear to be a bypass option within the plugin (making A-B comparisons even harder). In terms of stereo imaging I can hear what it is doing - 'pulling in' extreme panning to a slightly more central bias. I already know that stereo image is more obvious on headphones, and I'm not sure that is a problem (for me) when I'm mixing on headphones. However, I'm less convinced about the 'room / speaker simulations' - all that seems to do is roll off some of the top end (to varying degrees) and make things more muffled? That's not my experience of moving audio from headphones to speakers - some may have slightly less treble, but others have more.
So conclusions from this?
I think I am convinced that headphones with (as close to possible) flat response are what I need. Sennheiser HD558 are a strong contender at the moment (but I'll be doing more research).
However, having a pair that are covered by Sonarworks / Morphit presets are vital to making use of those tools.
I'm currently leaning towards Morphit, due to the lower price and the better volume matching. And they have Sennheiser 558s on their list of presets....
The lack of 'simulation' may be an issue, but I think I'll look into that as a separate requirement - try to find something like Isone but with more useful sounding options....
Thanks once again everyone for you comments and suggestions - as always I've moved completely away from where I thought I wanted to go, but I feel I am now making a much more informed decision.
Cheers!