18"? Do you sit on the floor? I used the 26" stands from them. Have you checked out the next level down in terms of stands from 'Digm, the S-series? Should be only $100 for the 26" ones. They aren't as nice as the J-series, but they'll do the job and the 26" ones are fillable. The others can be filled too, but have holes in them for passing wire through so you have to plug the bottom hole to fill them. The S26 has two supports, you simply fill one of them.
Even better IMO, the high mass stands at Parts Express:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=240-744 (also available in 18" height for a few dollars less).