The 9mm Xcel LX is well matched to the scope and gives a generally useful magnification in the 6SE. I used to find that my 6SE cooled quickly (and I used to live in a much cooler climate than here!), so it sounds to me like the seeing was poor and/or the local thermals, as other guys have mentioned.
The 6SE was my first scope and I had great views of Jupiter, Mars and eventually Saturn with that scope and it'll show you plenty on a decent night
