Remove the current 45Hz LPF (EQ menu) and add it to the ouptputs (in the X-OVER POINTS menu).
It allows you to set low end and high end XO points for the output selected. Mine, for example, looks like this:
OUT3 X-OVER POINTS________2/8
TYPE: But24______TYPE: But12
FREQ: 20Hz_______FREQ: 70Hz
That's basically a 70Hz, 12dB/octave LPF on the high end.
The left one you want TYPE: OFF(mine is music only with 20Hz port tune subs - so I'm HPFed in the XO at 20Hz), and the right one set up like the LP EQ was to confirm it gives you the same response. (Butterworth, Linkwitz-Riley, and Bessel will each give a different curve, even at the same dB/oct slope -- I'd start with BUT12 at 45Hz.) Then you can play with slopes to see if there's one that flattens the high end of your curve even more.
Your receiver will still handle the system crossover at 150 Hz, but I am also interested in seeing a sweep up to 200Hz, without mains, in your current configuration. With a 45Hz LP EQ, I don't know why your response wouldn't slope off to nothing before you ever reach 150.