With a little finessing and tweaking of the spikes, it should be very stable through the carpet and into the concrete.
You need spikes that are long and tapered sharply to get through all that padding. I made some platformes for my speakers with adjustable spikes because I have carpet on top of wooden floors. The platforms gave it some mass to clean up the lower end. But with concrete floors, that should be plenty of mass.
If it takes to much of the fullness out of the bass, just inch it closer to the wall to help fill it out.
Take small steps so you don't get to much bloom in the bass. It's a PITA, but when you nail it, all will be good.
You WANT the spikes to 'drill through' to the concrete. The key issue being even pressure downforce on all four spikes. You would need to find the right spot for sound from your speakers, Then check the spikes after forcing them completely through the carpet and pad to the concrete surface. and check the way they are balanced. If you have to add a bit of thin washer spacers on one or two spike then do it. (Even concrete floors are not usually perfectly flat) Keep at it until they all match in the weight they carry. When they all are balanced you will find the speaker really vibrates less. When i had B&W 805S on stands, getting the spike pressure balanced across the four spikes just right made a difference. The cabinet stopped vibrating! And the music sounded cleaner.
If you have adjustable spikes it is much easier, but even with non adjustable spike you have to add washers to it is well worth the effort.
PS: (even with four footed equipment with stiff chassis... to check those take a slip of paper and see how easily it pulls from each foot when in position. If one foot carries less weight that paper will pull out easier. add some thin paper shims under the foot (between the foot and chassis) until the feet all have the same 'slip' Then the chassis will vibrate less. and sound better.) a free tweak.
I have had good luck with spikes over concrete,with an area rug under the speakers and 1" thick Maple between rug and spikes..Spikes grip into Maple and are stable with the weight of the speakers holding everything down..The Maples is just cut 1" thick, 2"x 2" square under each spike.....
Speakers with four feet are a PITA. For all my previous speakers, I had custom Sound Anchor stands made, and those guys know the benefit of a three foot base. There is no need for the constant adjustment. My current speakers came with Sound Anchor bases the manufacturer has made for them and unfortunately they have 4 feet. Fortunately, they are easily adjustable and lock into place, but still I find the need to occasionally go back and readjust, maybe because the spikes are drilling into the concrete just a little over time under the over 100lb weight. If you have an option, and you can make the speakers stable with only three support points, IMO that is the way to go. As to your question, I have tried SA cone coasters, Audio Point cups and spiked directly into the floor on my carpet over concrete and I think spiked is best. On hardwood, I like coasters or cups with Herbies Big Fat Dots under them.
How heavy are your speakers? How thick is the padding and carpet? Have you tried your speakers with and without the spikes?
I also have a basement floor with carpet and padding. Over a 6 mo. to 1 yr. period I tried various combinations of spikes, wood plinths, rubber or composite footers, and nothing. This was for my VSA VR-4jrs. The worst was spikes directly through the carpet into the concrete. Grey, harsh sound with severely attenuated bass. What I finally settled on was a pair of custom maple platforms, 1" thick and about 2" extending beyond the speakers' plinths. Speakers spiked into the platforms, platforms spiked into the concrete thru the carpet. Got back my mid-bass, balanced out the rest of the spectrum, lost the harshness on top. I went with 4 feet on both sets of spikes but I have no doubt 3 would be better; also a 2" thick maple platform would probably be superior too. The platforms elevated my speakers a bit but luckily this did not cause problems re tweeter height and so forth.
I'm not sure what the objective is. Spikes are usually needed to decouple speaker from vibrating floor (infinitely small point cannot transfer energy). Your floor is concrete and most likely won't vibrate, so I assume you want to suppress speaker resonances. In that case you should couple speaker to the floor - either by direct placement on the carpet or on heavy base (thick marble or granite).
Spikes couple and allow vibrations to transfer. I realize that many theorize that spikes decouple by reducing the contact area between the bottom of the speaker and the floor. The reality is that the small contact point creates a very large psi value that in effect bolts the two components together. Vibrations travel very easily through spikes in both directions. Spikes are used with speakers to create a stiff, no rocking stance. All speakers are not the same so many times you have to try different things to find which works best for a given situation.
Wow - Someone put a lid on this open can of worms...=)
Thank you for all your input - I will give a few ideas a try: Im thinking try the Spikes to the concrete and see if that works. At 50# each I just did not think the spikes would "penetrate" into the concrete and perform properly. The idea of the spikes is to hold the speaker stationary. I was afraid they would "slide/vibrate" around. I will update on how much a PITA it is or was not...and my conclusions
I have a concrete floor with vinyl flooring and a rug over it. My Gamut L5s come on a base with 4 large outrigger cones; they also have cups for the cones to rest in. My two friends and I have listened to them several times with and without the cones resting in the cups and everytime we decided that they sounded better with the cups. YMMV.
I've had good luck spiking , only time it made things worse was on a springy floor that seem to resinate . The spikes will not harm the carpet .
Mine is a tad different still. Just berber carpet glued to the concrete floor, no padding. My Paradigms need the spikes to sit solidly and they sound better with the spikes, no question. My Aerial 10t, with the Sound Anchor stands mind you, sound the same no matter if spikes are used or the heavy rubber feet. My horns are too big to care. In fact, the bass horns were designed to acoustically couple with the floor so they need to sit flat.
WAVE KINETICS 2NS Loudspeaker Interface System
These will solve your problem & give you the results your looking for.
My highest Recommendation.
Ended up using the extremely sharp spikes on the front 2 post of each speaker. While using a not so sharp on the 2 rear post.
The front drilled through to the concrete and the other 2 rear ended up not going so far....
Got them all balanced and they decoupled the speaker from the floor and prevented unwanted damping.
Really sweet sound.
Thanks for all the adavise....