Subwoofer Isolation, what do you think...

I just got this big honking subwoofer, a DD-18 Velodyne. I've a ceramic tile over concrete floor. I also have a nice turntable in close proximity to the only WAF approved site for the big honker. Anyone have any feeling for if the adjustable feet that came with the DD-18 are enough or should I replace them with cones or something else? I always use the sub even with the vinyl as it adds so much depth to everything. I can't say my turntable's stand is very good, it isn't but I'm working on finding something good (and affordable!). I am concerned though with this big thing so close... maybe within four feet.
Thanks People!