hi Tim, i would be careful with the combination of the Grand Prix rack and the Basis 2500.....the fluid suspension of the Basis and the 'sloppy' suspension of the Grand Prix will probably not work well together. i don't know that for sure but when you are combineing 2 different damping systems in the same directions you will introduce a certain amount of instability to the Basis. the Basis 2500 is usually somewhat 'rackproof' meaning that it doesn't much care what it sits on.....but it needs relative stability for the fluid suspension to work properly.
i tried a few different things under my 2500 and couldn't find anything that made any particular positive difference. you might consider tradeing the fluid suspension for the solid legs if you want to get creative with racks.....i never compared the 2 approaches but i would think the solid legs with a vibraplane would be killer.....but in general using two different suspensions on anything is not advisable.
imagine a car riding on 2 different suspensions......a tire-guy like you would be replacing the tires all the time. sorry, i just had to say that.