Hi-
I am in the middle of stems, can't rap too long...
I think you have a magnetism problem.
There have been some helpful hints here but in case they were vague:
1) TAKE YOUR SESSION OFF OF THE DECK!!!!
2) Record something on to another piece of tape, scrap if you have it, 10khz will work well and note on the meters of the recorder where they are. For this it is sound to record at 0vu. Record at least two minutes of this so you have enough to experiment with (turn your monitors down).
3) Rewind the recorded section and play it back and see if the material is quieter in sync or repro.
4) With the tape playing push the tape agains the head a little harder by pressing into the moving tape with your finger next to where it hits the head, but not on the head. See if that causes the high end to come back. Like previously mentioned, that would signal a tape path issue.
5) If this is not changing it for the better play the tape over and over where you have recorded your 10khz tone about ten times and see if you are getting loss in level on the vu meters. If you are, you almost certainly have a magnetism problem.
6) If that is the case, very serisouly learn how to demag your machine. A magnetized tape path will cause high end loss much more prominently than mid and lower frequencies. What would also lead me to believe this is your problem is that you complain of phasy souding high end. If you demag improperly, you are going to actually magnetize one spot on some part you have tried to demagnetized and if that part is a rotating part, as that part rotates past a section of tape the amount that it removes signal it removes will vary with how strong the magnetism of the part is.
If you picture rotating parts from the top down and assume 12 o'clock is magnetized, as the part spins around (like an idler) it will suck high end whenever the 12 o'clock makes contact with the tape, with less damage occuring as you approach and as you pass 12 0'clock.
Also, you do not need to move the demagnetizer more than a few feet away while you are slowly back the demagger off the machine. For whomever that was that moves 15 feet away. Also, when demagging, most of your demagging happens logarithmically as you close in the metal parts. SO, back away verrrry slowly for the first inch or so as you waive the demagger from side to side.
Get yourself an "Annis Han'D'Mag". They are the best.
Cheers and good luck,
josh