what do you put under your sub?

Air.
I have two 18" infinite baffle subs which are ceiling mounted. They are in
plane with my mains. Unbelievable.

Your Sub should couple to your floor. It should not be isolated. Spikes in carpet and original feet on other surfaces.
Cheers.

12"X 12" landscaping pavers.

I was laying on the floor next to one of my subs while the music was playing and could feel vibrations from my janis subs. Also the cables are directed out of the bottom of my sub. A quick check of the Soundanchor website and Alberporter revealed that they have stock stands for subs and also make cutom stands.
I contacted them. Sent them the measurements for my speakers. Sometime later a set of stands arrived via UPS.
Problem solved. Carpet no longer vibrates and the sub is no longer sitting on the cables. The bass has tightend up considerably.
BTW I support and encourage DIY solutions to problems like this.

I have two solutions. Under my VSA sub in my HT room, the sub which is a downshooter sits on spikes on a 2" maple block from Mapleshade. The block rests on four (4) Isoblocks (cork and rubber also from Mapleshade) which rest on the floor. This is a suspended wood floor which is hollow underneath since there is no slab. Bass is tight and there is no boominess through the floor.

In my two channel room upstairs, the MJ Acoustics sub (also a downshooter) sits on Nordost spikes, which in turn sit on an Auralex SubDude. Check this out, Auralex developed this design for roadies to use to isolate their speaker cabinets from hollow temporary arena stages. Inexpensive and very effective - bass is tight and there is no annoying thump through downstairs.