Yamaha DTXpress/DTXplorer/DTXtreme

Has anyone encountered a problem with the clamps attached to the rack 
or holding the pads?  I've got a clamp "screw" that will not loosen 
regardless of how much force I put on it.  It will tighten, but will 
not turn to loosen.  Don't know what's wrong with it.  Also, since 
the set is still under 1 year warranty, I assume Yamaha will replace 
it.  Unfortunately, I use it for church practice Wed night and for 
church on Sundays.  Any experience with Yamaha timeliness on warranty 
replacements?  Can I buy another brand and then deal with Yamaha? 
What other brands will work?  By the way, I have noticed other clamp 
screws seem to be slipping.  This may be the start of more clamp 
problems.  
Thanks.
Bill