Hey Fred,
You should probably give us a little more detail if you want something other than generic answers.
In particular, you should be more specific. For instance, you should explain the procedures you're followed when measuring the freq. response.
Do you have a good test tape? If not, you should get one. Otherwise, it's hard to figure out whether the issue is on the recording side of things or the playback.
If you get a test tape, you can also check the head alignment. No amount of bias or eq can overcome heads which aren't aligned. You mentioned that you have a new 4 track record head but just because it's new, doesn't mean that it's adjusted correctly.
There are many adjustments other than the head alignment on both of your machines. Almost all of them can cause high frequency issues and are (for the most part) getting the tape to travel across the heads correctly.
Stuff like tape tension, pinch roller pressure and keeping the tape from being pushed against either side of all the guides/tensioners effects high frequency response.
If the 1520 is new to you, I'd suggest that before you start to change anything, you need to go through and service all the mechanical elements. One of them is more than likely, the problem.