Effects of concrete as base for subs, speakers and potentially equipment

I limited my earlier response to your question about using custom concrete slabs and did not address the floor. The wider the slabs, the greater the improvement by spreading the load over a larger area, but @o_holter is correct that gains can be made by reinforcing a suspended wood floor from below.

The easiest way to improve that situation would be to stiffen the floor by attaching a simple, adjustable jack-post column beneath the floor joist nearest to the two speaker locations and possibly also near your equipment rack, as @o_holter did near his turntable. However, not everybody has the ability to feasibly add support columns in their lower level, depending on the usage of the room below where your system is located. Some folks have reported an improvement by adding cross bracing between the floor joists in the vicinity of the speaker positions, with or without the support columns. If you do add cross bracing, be careful not to affect the integrity of the floor joists by cutting or drilling into them (consult a builder if you need advice).

In general, structural members are stiffer near the ends where they are supported so the closer your speakers and electronics are positioned to a bearing wall, the less of an issue you should have.

I am going to base this on our vibration control at my workplace.  We measured vibration in all frequencies.

You buy a speaker and put it on the floor.  Doesn't matter if it's wood or concrete although wood can flex more.  As the speaker plays it essentially bounces around throwing its sound waves out of phase.  

I think concrete is fine because of the mass ratio vs the speaker, but spikes into the concrete still allow the speaker to vibrate and move.  Another piece of concrete on top of the floor then bounces.

A floor that isn't level also causes phasing issues.

Try a set of $30 springs first. Audiocrast and Nobsound make them, 4 per speaker.  Herbies makes some special silicone also. Either of these examples dampen the items and greatly reduces movement.  Springs are self leveling.  My floor was not level. You can go on up the price scale real soon.  If you try these low cost solutions first and decide if you want to buy something more pricy, or stick with them.

I went to the countertop place and got scrap granite and marble to use under my speakers.  I thought it helped.