Try excell mains at 56 lbs and lux pwr rough crosses at 52 lbs. You could even go lower.
It will feel as stiff as all excell at 60 lbs.
I have gone thru the same thing over the last 6-8 years and my experience is a hybrid with a poly cross can and should be strung low. I was using multi's at 58 lbs and now use multi/poly between 48/52 and 52/56. I am thinking of trying lower. I started like you suggest with the multi 58 and the poly 3-4 lbs less but developed a bit of an elbow problem. Dropped the tension, and elbow is no longer a problem.
Poly will still allow you to maintain control. Especially in your k90, you should be able to string low due to smaller head size.