Your location may play a variable role. On the one hand, humidity can add a haze. On the other hand, meadows and woodlands cool down and reach thermal stability sooner than concrete and asphalt, and that is in your favor. If you happen to live near terrain that causes turbulent air flow above you (and that's hard to know), you may have a rough life.
A few more hints: first, be sure your scope arrived in good collimation. Once it has cooled for a good 45 minutes, look at a bright star. Bring the star to focus, then just slightly out of focus, so that the image shows a central bright spot surrounded by one or more rings. Are the rings perfectly round? Is the central spot exactly centered inside the rings? If not, then you need to collimate the scope, and that can make a big difference in how much detail you see. If you're out of collimation, then your eyepiece isn't looking at the best image that your scope can deliver.
Second, try to position the scope so that it isn't peeking right over the top of anything that would make heat currents (i.e. Jupiter barely above a roof top). You'll get your best views when the planets are well above the horizon.
As for magnification, on most nights an 8-inch scope will work reasonably well up to 200x or even 250x, and you can see plenty of detail at 150x. Even at 75x the main bands on Jupiter are visible. Only on the very best of nights will it work beyond that (but such nights are a reason for living). Your Barlow may serve you well one or two nights a year, and you'll remember those nights, but 300x or 450x is higher than you can usually use because the atmosphere is too turbulent.