Unfortunately I can't tell you how the 1840s compare with the HD800 or T1 - but I can give a brief comparison with the HD600. The 1840's are lighter and have stronger build. Very comfortable - but headband cushion still needs some work. Tonally the two are very similar. Both quite neutral - but HD600 remains the more natural sounding in my eyes. SRH1840 is very flat - with no real mid bass hump and better extension into the lower bass. It also is slightly brighter than the HD600.
Soundstage is similar - but the 1840 does have a little extra clarity and separation, and IMO images slightly better.
For overall value though - the HD600 remains king. If you can pick up an 1840 for the right price (I saw one in the FS recently at AUD 450) - then it's worth getting.
Not sorry I bought the 1840 - have thoroughly enjoyed it so far - although probably looking to move onward and upward as soon as I can afford it. HD800 still beckons.