As always, what follows is very subjective....
One year ago I ordered the T1s after exhausting everything I could read about ACS. They had to go back for a couple of re-fits due to poorly made impressions....customer service was exemplary.
I have to be honest......I really didn't like the sound. I found that a particular mid range frequency sounded exceptionally harsh. I suffer from mild tinitus (too many years playing jazz trumpet next to loud PAs) and the very pronounced mids were actually painful to listen too for all but the smoothest acoustic music. For example the vocals and guitars listening to bands such as Muse and Kings of Leon were soooo pronounced it spoiled the whole listening experience. In all other respects they sounded great (beautiful seperation, deep bass). Anyway, I soldiered on with them (£649 is a hell of a price to pay for something you don't get on with), persuading myself that I would get used to them.
A couple of months ago, the right channel stopped working so I sent them back to ACS. I spoke with Matt (from the technical side of the company rather than customer service) and told him about the harshness I had experienced. His response was unequivocal ......have them remade as T2s!! To be exact, have them remade with the Knowles build (ACS use two driver manufacturers for T2s, Knowles and Sonyon.) He assured me that I would be much happier with the sound sig of the T2s given my particular experience with T1s.
The T1s (as others have eluded to) are primarily for stage monitoring and as such really bring out the frequencies that are required for this purpose. I now have the T2s and I have to say they sound much lusher and creamier than the T1s ever did. They still have the same separation as the T1s but the bass is more pronounced and the mids more recessed (still very defined however)....exactly what my ears had been craving. They are so much better for rock, classical and Jazz (IMO).....the only place they miss out is the absolute clarity on acoustic vocals that the T1s managed very well (You really could hear the singer's every nuance.....(lip licking etc)). I have to stress though that this was only pleasant to listen to on the most stripped back acoustic performances.......anything more complex (add in an electric guitar or 2, maybe some mid range piano) and it became jarring to listen to at anything other than low volumes. I felt I was missing out on everything else that was going on as I couldn't get past those damn mids!
It was an expensive way to find out that, for me anyway, the cheaper product is way more suited to my ears. Of course they didn't refund the difference...not that I expected them to!
BTW.....I'm going to post this as a seperate posting as I think others may benefit from this experience. I would hate people to spend more than they need to! Aplogies in advance for the duel posting