I'm assuming by the low pass filter you mean gaussian blur (that's what its called now in Photoshop). I guess the clue is in the name. It is effective in removing noise but also will take the edge off your edges. Following this with a sharpening tool (you don't say which one) is not likely to get good results as effectively you have reduced the all important edge detail which the sharpening tool needs.
This is of course a generalisation - gaussian blur
CAN be very useful in some cases (eg. portraits) in order to reduce fine skin detail (pores, spots etc.) But not really appropriate to use where crispness is desirable.
Unless you are using a very high ISO setting you should not be getting excessive noise. The most likely cause for this is underexposure, so I think you should look at your technique for the initial cause. Trying to fix a photo in post processing is generally a losing battle. The initial image should be sound, and post processing used to touch up the results.
Try bracketing your exposures next time you are out shooting - try a stop over your normal exposure and see if that helps with your noise problem.
I think you've already proven that equipment is not the issue!
Cheers,
Colin