Ben, with barlows, as you might know, the barlow increases the magnification of whatever eyepiece you use by the rated amount inscribed on the lens (x2, x3 etc). This is all well and good, but what you have to take into account is just how much magnification can your scope actually handle. There are a couple of quick and dirty equations you can use to determine this...diameter of mirror/lens in mm x f ratio / ep' lens diameter, or, diameter of mirror/lens in inches x 60 (which gives the max mag your scope can handle). Since you've bought a 6" f8 scope, using the smallest ep you have (10mm), the max mag for your scope is ((150 x 8)/10=) x120. Now, the highest mag your scope can handle is around x360, however for various reasons, for a 6" scope this is impractical. Mainly to do with the conditions you observe in and quality of the optics of your scope. For the most part, with a 6" scope I would be reluctant to push the scope beyond about x180, and even then I would be hesitant to use such a high power. Even in the best of conditions. So, if you have, or are going to purchase, a barlow, don't go beyond a x2 barlow and don't go less than a 10mm ep for your setup. A 10mm, 15mm, 25mm and 32mm set of ep's would suffice and give a good spread of mags.
Hope that helps
