Its always better to use a reference disc like Avia. Besides level adjustment it has some good phase tests so you can also make sure the phase switch and level on the sub is optimized with the speakers.
In setting the crossover its important to remember you are trying to find the frequency which yields the smoothest response between the speakers and the sub. The room and position of the sub and speakers will be an significant factor in where that will be. A good place to start would be a half octave above the +/-3db low extension which in the case of the 60s is around 1.5 x 37hz or 55-60hz.
I like to use single sine wave tones 10hz on either side of the crossover or alternatively the LF sweeps available on the Avia disc in the verification section. You're aiming for the crossover point with the least volatility on the metre. Also check at 50hz, 70hz and 80hz, It will vary depending on where you have your sub positioned and your unique room acoustics. When you finally settle on crossover frequency, adjust the phase so you get the highest SPL then go back and recheck the measurements for the crossover to make sure.
Here is one of the better articles on using the RS metre and Avia to calibrate speakers and sub.
http://www.bigpicturebigsound.com/article_560.shtml
