Jedinite:
I have a similar setup to yours. I have B&W 803ds that have 2" Eden sound footers, mounted on a 3.5" thick maple plinth. Under the maple plinth I have 3" diameter Eden Sound long spike carpet footers that pierce my carpet to a wood floor. This system sound fantastic, much better than footers alone or stock B&W spikes alone.
I would not eliminate the plinth, as this acts as a second stage of vibration sinking to your floor. If you go footers right into the wood/concrete floor, I think your speaker sound will degrade considerably, as concrete is the worst material to sink into (as it reflects sound back up). You can try discs under the plinth spikes to protect you floor, but I am not sure how good those will sound. Is the engineered wood thick or is it the 1/4" Pergo type?
You might also want to talk to the folks at Maple Shade. They can give you some tips about mounting plinths over various flooring materials.