Any alternative to Radio Shack sound level meter?

Kijanki all that is wonderful but now you need a very high resolution way of digitally creating mirror images of the curves then EQ on top. It takes at least a 64 bit system. The best way is with a digital preamp with room control and the ability to modify curves such as the Anthem STR and Trinnov Amethyst. 

I have a PCE 353 which I used to set up a pair of Wilson Benesch Torus Infrasonic Generators with their outboard amp/crossovers.

https://www.pce-instruments.com/us/measuring-instruments/test-meters/decibel-meter-kat_162375.htm

You only need to match the sub bass drivers to your main speakers 1kHz level using an appropriate test tone found on many reasonably priced CD demo discs. 

Use your ears to fine tune to taste. It is not as difficult as many lead you to believe.

I hope this helps!

Thanks for the great responses. I have already done the simple setup a couple of years ago, with the xover at 50 hz and 24dB with the level set by the RS meter. But I want to set up the PEq, so I may need the precision of the option by Kijanki, and I as I have a 64 bit PC close enough to work, I will probably try that.  (My smart phone is only a cheapie Samsung.) Thanks again.

@oldears  I did few frequency sweeps only, so far (results  look pretty bad), but program has a lot of tools:

 It includes tools for generating audio test signals; measuring SPL and impedance; measuring frequency and impulse responses; measuring distortion; generating phase, group delay and spectral decay plots, waterfalls, spectrograms and energy-time curves; generating real time analyser (RTA) plots; calculating reverberation times; calculating Thiele-Small parameters; determining the frequencies and decay times of modal resonances; displaying equaliser responses and automatically adjusting the settings of parametric equalisers to counter the effects of room modes and adjust responses to match a target curve.

The easiest I've used is a free download app called audiotools.  It provides a sound level meter and also real time analyzer adn fast fourier transform tools.

You can see gaps in your system and room.  I use the stereophile test cds and play pink noise or other signals to see where the system and room is lacking.

not complicated at all and quite easy and simple to use.

Not professional quality because you are using your Ipad or Iphone's microphone, but it works pretty well non-the-less.

enjoy