Go for 940! It is flat and very detailed. In fact, they are the most detailed from the entire SRH line(including SRH1840).
I currently have the 840's and had the 440's (they were broken by a child).
The 840's are better in every way, it sounds much more natural and less digital sounding. The sound is warmer and BUT amazingly has more detail than the 440's.
Songs that were piercing and harsh are now tamed.
Songs that just didn't have enough bass, authority, and mass are brought to life.
I also noticed that vinyl sounds in tracks are much louder and clearer ( the popping, grainy sound), which is great.
Comfort wise they are pretty good, at least to me. I used to wear the 440's for long periods, and I could were the 840's all day.
Some say that the 840's are veiled, if you feel this way, I did a test buy swapping 440 ear pads to 840 headphones.
The 840 pads driver cover is thicker than the 440's buy a good amount.
The results
A large treble and mid range boost, the mids were so powerful that I could feel the vocals resonating in my head (which was a first).
The experience was great.
In short the Shure SRH840's with SRH 440 pads sounded like the 440's but clearer, bolder, and natural. There was also a loss in bass quantity with the 440 pads.
I got the 840's for $160 total used on amazon, it included all accessories and a free straight cord.
The SRH840's are a clear upgrade!