I a few things:
Your input power?
Plate current for 600 watts out?
Where is the load control?
You say as you drive it harder the grid current goes up - is there no more adjustment left on
the load cap?
It would be interesting to know what your results on 10 meters are - since you could have a tube that is going soft.
I have used about 5 different AL-1500 amps over the years. I have two at my station now that I use in my SO2R setup. Several of them - including the one I have owned the longest always took more drive on 15 meters.
When I got my second AL-150 a few years ago I noticed that it was much better on 15 meters. One day I tuned them both up and looked at where the caps were set. On the one that was harder to drive I noticed that the load control was at or near the limit (I do not recall if it was fully messed or the other way around) but there was almost no more adjustment left. I think indicating the inductance needed to be adjusted. I opened both amps up and noticed right away that the coil on one had turns much closer together than the other - on the one that was taking more drive - so I spread them out. The next thing I noticed was one end of the coil was bolted to the cap in a different location - like one was on the outside of the cap and the other was on the inside. The difference was not much but it was different. So I made them the same. These small adjustments made all the difference. The net effect was that I increased the inductance on the one that was taking more drive to get full output.
If your loading capacitor is not in the mid range I would look at adjusting the inductance.
But first I would make sure you have a good tube.
There are ways of tuning the tank circuit of an amp without high voltage on it using a resistor and an antenna analyzer - you can tune your tank circuit.
As outlined here:http://tinyurl.com/dyxacbo