I'm currently looking at upgrading my iPod earbuds to something a bit more high end - specifically, the Etymotic ER6i, but wondering if the sound is comparable to the more expensive and full headphone sized Sennheiser or Grado? I'm talking purely on a subjective sound quality basis, would one type be preferable on another for critical listening?

This depends on the person. Some people use etymotics, especially the ER4, just for the sound. I think they are a minority though. Etymotics (and other types of in-ear phone) are good for isolation and also sound good to many people but if you don't need isolation probably other headphones are better choices. One advantage of the ER6i is you dont need a separate amp since the frequency response is specially tailored to cancel out the flaws of the ipod internal amp.

I have the Etymotic ER4-P, Sennheiser HD600s and HD25.

In a nutshell the ER4-P can't in anyway shape or form match the Sennheisers for sheer listening pleasure (for my ears, this isn't fact!).

I can't listen to the ER4-Ps plugged directly into my Transit or indeed via Audiophile 2496 > Arcam A85 chain. It's quiet frankly horrible. Too bright (so much so it almost hurts) and probably as a result of that (it won't help), little to no bass.
But, don't let that scare you off, haha. I've tried the ER4-Ps plugged into my mates Ipod Mini and like my W800i it's a much more pleasureable experience and they're the best earphones (not headphones!) I've heard. I had to tweak the EQ a little to help them along though.

Please, please take my comments with a pinch of salt. If you can, try some, the experience maybe completely different for you.

I was disappointed by the ER6i with my ipod, and was looking to buy ER4Ps, and buiding an amp. I hope that should work out, but for now, I enjoy the sound I get with my Senn HD280s more than with the etys.

I used to have an ER-4S, and IMHO it sounded better than my HD-600. The trick is that they are very sensitive to how they're inserted into the ear canal. If they're not in far enough and/or don't have a proper seal the bass will suffer tremendously, causing them to sound very bright. Putting them in too far is also bad, both for sound and comfort. I ended up getting rid of them because I no longer needed the sound isolation they provided (got a different job) and at home it was, IMHO, annoying to be effectively deafened to the world while wearing them.

If you don't mind the way they fit over your ears, the Koss KSC-75 headphones are very good, along with being very cheap, and they're even liked over at Head-Fi, home of the rabid headphone audiophiles.

I can confirm this. To my ears they are very good too.

I have Etymotic ER4-P, Sennheiser HD25, and Grado 325i. I love the Grado's, but can't use them often because of the sound leakage which drives people around me mad. The Ety's I also love because of the isolation, but they do need serious eq for them to have enough bass for me. I've got Iaudio X5, and the Machbass does a grand job at this Still usually return to the HD25's though, as they have such a wonderful "slam" to them.

Plugged into my A85 they can blur my vision. Not healthy, I know.

Hmm, well I already got the black version of http://www.etymotic.com/ephp/er6i.aspx

I'm sure i'll be happy with them though, coming from standard iPod earbuds (which i didn't think were as bad as everyone makes out if you use them with the included felt pads)