I have been doing some research via the Web on headphones. From what I have found, the Sennheiser HD600's and Grado RS series is the level I am looking for.

I would like comments from others that have also investigated these and others. I have had no success at finding them locally so I can try them for myself.

I am also interested in a headphone amp, so I would appreciate advice on those as well.

Lastly, I am also interested in the following combo, AKG K1000 headphones and the SAC K1000 amplifier. Has anyone had any experience with these?
I have both the Sennheiser 580's and 600's, the 600's are superior in midrange clarity, bass response, and comfort. Terrific headphones. I've used the 600's with all kinds of different gear, so far my favourite is a Grado RA-1 headphone amp. I'm sure many have had good experience with Headroom and Antique as well. Can't go wrong with 600's.
I just bought a pair of Sennheiser 600's last week for $250.00 new from hififorless!

I compared them directly to my Grado 200's (now discontined, close in sound to the hp-2's). The Sennheiser's are very sweet sounding in the top end, and are way less colored than the Grado's. On certain dynamic music, I sometimes prefer the Grado's, but I am always aware that this is a false and colored rendition of what the recording actually should sound like. Ironically, I was not aware of the Grado's shortcomings, until I purchased the 600's. The 600's are a true reference product, that at $250.00, are unbeatable for the price. (Note: The 600's also improved dynamically after about 48 hours of continuous playing)

I should also note that I owned a pair of Stax pros years ago, and miss them much more than I ever had imagined. These were $1200 headphones (at that time), that were not perfect in terms of ultimate low bass slam, and their highs could be a bit overbearing at times (this is why I eventually sold them).
Unfortuntely (and ironically) , neither the Grados or the Sennhesiers are as ultimately enjoyable to me. There was something about that electrostatic midrange purity, maybe it was the speed, that I really, really miss (regardless of the other pitfalls).

Good luck on finding the best headphone.
I have the senn 580's. I think overall they are the best I've heard. I do note that they have their drawbacks. They tend to sound a little 'too smooth'. Sometimes they are also a little 'too good' and tend to overpower you with detail. I find that at work I sometimes prefer listening to my old sony v6 studio headphones. They just have a relaxed quality and are a little easier to listen to as 'background' music. Some other bad characteristics of the 580's are that they tend to sound loud to others in the vicinity, and they don't have any sound deadening ability. You hear EVERYTHING going on the room around you. The sony's are much better in this respect.
I own Senn 600s now and have had 580s and Grado 325s. I love the 600s. They are much more comfortable than the Grados, and I think more sweetly detailed as well. I found that the Grados had a bit of grunge that never allowed them to open up like the Senns. The 600s are a good step up from the 580s as well. I have used the Senns w/ Musical Fidelity xcans v2 amp, and it is a very transparent experience, though with out as much bass slam as you might get with one of the more expensive headroom amps. Good luck!
..this would come under the "For what it's worth" catagory.. I had worked in the audio industry(?) at one time, and had several Stax of note.. I then had a pair of Audio Technica electret condenser headphones...and have never liked any better. Sold them long ago, and had tried the top end wood body Grado...but nothing got me into the music like those old Audio Technica's.... A friend had said that Stax had the same model, and that Stax may have made them for Audio Technica.
This may be of little use, but does anyone else have any experience with the AT or Stax Electret Condenser models?
I own Seinheiser 580 and have listened to the 600 (which are much better than the 580). I do not have any experience with the Grado. As to a headphone amp, either headroom amps or melos tube. I really like tubes with headphones. However, I would recommend something a little different. Stax headphones, preferably with the tube option. On the used market you can find them near what you would spend on the 600s and a good amp. I recently bought a pair of Lambdas with the solid state amp for $450. If you plan to do a lot of listening with headphones, the Stax are really the most enjoyable over a long period of time. Great clarity and detail and not so "headphone" sounding. One thing to keep in mind they are "free air" headphones--so others in your vacinity will hear some music from them.
Sorry to stay off the subject, but Whatjd, I still have the AT electret headphones. I have not used them in years, but once upon a time I was very very smitten. Very engaging headphone. The problem at the moment is that they almost require that you have a balance control, since the manual tells you that one channel will likely be louder than the other, and mine do this to a certain degree. Bought them around 1980, but never a problem. Just thought you might be interested.
I have the 580s which I use with my portable audio with the entry level headroom amp. Definitely better than the 500 I used previously. I tried the 600s in the recent NY show with several amplifiers and despite the reviews I was not sufficiently impressed to buy them. In my bedrroom I use the Beyerdynamic 931 which are excellent and so far not reviewed in USA. Amp is Wheatfield HA2 but the much cheaper Antique sounded good also. I find that the 931s reproduce very well the extremes of the audio spectrum while having an accurate midrange.
I have and really enjoy the HD-600's. Excellent for Jazz and Classical as well as accoustical music. Very comfortable. If you like aggressive sounding (forward placement) sound (especially rock) and can stand the way the Grado fits on your head (I find it very uncomfortable), that may be a better fit for you.

The typical comparison between the two is generally as follows:

Listening to the Senns is like listening in the 10th row center of an auditorium. Listening to Grado's is like being on stage with the band/orchestra. Grado's are very dynamic but I find them a bit fatiquing and a bit colored. Others find them exciting and the Senns too layed back.

As for electronics, I greatly prefer tube Amps with my headphones. The MG-Head by Antique Sound Labs is a very good buy at about $275.

Good luck.
The best thing you can do is to audition as much stuff as possible.

The HD600 is really nice but I prefer the Grado RS-1. I don't like way the HD600 smoothes over all recordings good or bad. When the album has bite, I don't want a filtered out kind of distant rendition. You can get a very comfortable fit by bending the headband and tossing the earpads in the washing machine with some hair conditioner.

As for headphone amps, they range from $99 to $5000. Poor to astounding. I didn't really find anything to my liking until I broke the $700 barrier. Lower priced tube amps were seriously flawed in one way or the other.
I have lower priced Grados and old Stax lamba's at home, so I can't help you there

If you are interested in headphone amps and have an interest in a great fully functioning tube preamp, try out the SOnic Frontiers Line 3, (or 2 or 1) it has the Headroom circuitry headphone amp stage and is one of the most neutral preamps on the planet. Great detail and neutrality in a fully functioning preamp. Check out the reviews on their website, the Soundsatge review included the Sennheiser interfacing with this headphone amp.
Headphones are a very personal decision as not only sound quality but comfort is involved.
You can spend hours reading posts here and at headphone websites debating about whether Sennheiser or Grado is "better" and will probably find no conclusive proof either way.
They are both good.
I have tried Senn. 580's at home and have listened to the 600's in various dealers.
However, my personal preference is for the Grado RS-2's.
I do not like headphone amps. I have tried 3 different models, tube, solid state and hybrid and none were much better than the headphone output of an NAD receiver. I should qualify this by adding that the NAD sounds better than the headphone outputs on any cd player or tape deck that I have tried. I have gotten more bang for the buck by upgrading headphones and my cd front end than from a headphone amp.
Although I had initially liked my old Staxes a lot, I sold them last year for $200 as I found I hadn't listened to them for years.
A recording engineer suggested that their uptilted response was great for hyperdetailed recording mixes, but way too fatiguing for extended listening. And so it seemed....
I recently purchased HD600s from a reseller in Germany for $225 (eBay), and find them MUCH more forgiving than the old Staxes. I drive them directly off my Pass Aleph P with a short $4 RCA to phone Y adaptor from the Shack. Sounds great, and they're super-comfy as well.
I'd like a few more feet of reach. Anybody out there know of a clean phone extension cord? I think Senn sells an additional 12 footer, but with a built-in volume slider, which of course won't do! Thanks. Ernie
If you need an extension Headroom.com has best selection. The DiMarzio silver at @$60 for 10ft extension
is very nice, I own one. Really you should look into getting
Clou Blue Jaspis to replace stock Senn cord, it is 3 meters
and looks and sounds great for $90. I recently bought one for my Senn HD580 and Garfish also just purchased one for his Senn HD600.

I have another thread started on Blue Jaspis where I will have to post more detailed info.