port size problem

The 1.58 cu ft is figured after you deducted port structure volume, correct?  If so, leaving the port as it is results in a tuning freq of about 56 Hz.  If you block up 4.5" on each side of the port, or 9" total, to reduce port dimensions to 6" X 1 7/8" and 11" length you will lower Fb to about 44 Hz.  I think that it would have to sound better with the lower tuning freq.

But since it sounds good the way it is, you might try this:  cut your blocking pieces and insert them into the port but don't glue them (just a dab of caulk to hold them from making noise).  Make your decision based on the difference that you hear.  A single 9" X 2" X 11" piece of shipping styrofoam would do the job quite handily for the test.

And here's a way you can use the port length calculator on this site to help you out.  When you know the slot port opening dimensions you can use pi R squared to find what the equivalent round port would be.  Use the formula in reverse:

15" X 1.87" = 28

28 / 3.14 (pi) = 8.93

square root of 8.93 = 2.99

2.99 X 2 = 6 (real close)

6" diameter is the equivalent of the port opening size you now have.  If you block out 9" off the 15" width, then equivalent 4.6 inch diameter is what you are looking at.

Use the calculator included with your computer to do the math, including the square root  step in the equation.  Anyway, that's how I did the math to get those results.  There are some other port calculators floating around cyberspace but I didn't want to look. 

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.