I know it violates your stated desire for a diaphragm pump, but I think the Marco gear pumps are the cat's meow.  I think you'd be happy with a #3 or a #6.

They are simple, robust, quiet, and very compact with continuously variable speed so supply pressure is constant.  We have had one for almost 5 years, and haven't needed to work on it yet.  A spare parts kit of a shaft seal, motor brushes, and a few gears and they should go almost forever.

I personally would not go to a 55 PSI pump.  That is just an invitation to waste water.  We are very happy with a steady 25PSI.  

Bill Kinney
SM160, Harmonie
Charleston, SC, USA