Should I build plinths or screw spikes into cabinet?

I've got a great old pair of B&W DM640 floor standers, but I don't have the plinth and spike kit that were sold as an option back in the '90s.  Since I have been unable to track down a kit online or directly through B&W I am planning on building my own.

The speakers themselves were built without any isolation system on the base of the cabinet, and were set on top of adjustable "lugs" which were incorporated into the top of the optional wooden plinths.  Each plinth had four basic spikes mounted at the corners.  This sounds easy enough to replicate.

Does it make more sense to simply drill into the base of the cabinet and mount the spikes directly?  The tweeters are nearly at ear level, so I don't need to lift them much.  I am a little hesitant to make any permanent alteration to the speakers, however.  Any opinions or alternatives are welcomed!
I think you need to know how deep the wood is on the speakers. Perhaps, a pilot hole. If they are deep enough then adding threaded spike inserts is the best way to proceed.
One option:
You need these-

The Townshend Seismic Pods or Bars suggested by infection are great, but not cheap. For as little as $75 for a pair of speakers you can try roller bearings.
Thanks everyone for the responses.

Jamnesta, I contacted last week, and their cheaper option (non-custom) still added up to about 40% what I paid for the speakers not long ago, so I'm still looking for a cheaper option.

Infection - Are these available in a cryo-treated option? Haha.

bdp24 - I'll do some research on that avenue.  I'm hoping to stay <$100.
Good luck Guitared, I'm sure you'll come up with something for your speakers that's cost effective. I have little experience outside of the spikes I was provided with my speakers, but your post brings up an interesting quandary. Couple to the floor with spikes or decouple from the floor via the Townsend or some other method as suggested by Infection & bdp24. I imagine that's been debated endlessly here and elsewhere.....
As your budget is ideally less than  $100 then obviously the Seismic Bars are out of the question! 

Perhaps you could fashion your own version...!
A DIY version of the Seismic Bars could be a pair of Baltic Birch plywood shelves separated by a very slightly inflated bicycle tire in between them. With a trio of roller bearings between the speaker and top shelf you will have excellent isolation without the non-linear filtering caused by "lossy" materials (rubber of any type). 
I've added spikes into threaded inserts, but I've also ordered DIY roller bearing setups.  The latter will likely end up being used to isolate my components since I'd prefer to reduce the risk of a loudspeaker getting toppled over (baby on the way!), but I plan on tinkering with both.  Thanks again everyone for the feedback!
After getting the speakers up on the spikes I had a quite significant drop in sound quality, particularly in the low-end.  I have suspended hardwood floors, and it turns out I'm suffering the same side-effects from coupling seen by many others.  I should have done more research...

I've got them up on ball bearings in furniture floor cups now (spikes removed), and I am hearing a great improvement.  I'm going to throw some stone tiles between the bearings and the spikes soon, then probably bump up to some vibrapods or feet/isolated spike cups from Herbie's audio labs.

Infection, I wish I would have investigated the Townshend website further instead of just joking about the price of the seismic bars!  They have a great video that explains the benefits of isolation vs. coupling, and I would have initially gone in a very different direction!
guitared---Barry Diament, the guy I heard about the roller bearings from, put his Magneplanar 3.7's on them, and uses slightly inflated bike tires between BB plywood planks for his electronics, even in his professional recording studio. Almost as good as the Seismic Pods, and a lot cheaper!
Hi BDP, I've done exactly that with each of my components and could not believe the improvement they made.  I'm using marble tiles instead of plywood, it was actually cheaper!  I also have marble tiles under the speakers and above the homemade rollers presently, and every aspect of the sound has been improved over just spikes on the hardwood.  I'm a little unsure about adding the inner tube under the speakers for stability reasons; the system is in a living room and I'm worried someone (or some dog) will knock them over.  

Has anyone tried this with a more traditional floor standing speaker?  I may get 2 more tiles and try sandwiching the tube or rollers between them.  The cabinets are rectangular so I need the tiles to allow placement of the rollers in an equilateral triangle.
I'm anti spike for speakers since putting Vibrapods under my Silverlines. Keeps them independent of floor vibes and they simply sound better than they did with spikes.
When guitared posted he reminded me of these: Ingress Engineered Products. With some flagstone or similar product on the floor it could be done.

All the best,

The Ingress are the best deal around in roller bearings. He now offers a version made to Barry Diament’s specs: a highly-polished single cup made of 7075 aluminum, with a large diameter, shallow bowl (the result of machining the shape of the bowl to follow the curve of a very small section of a very large sphere) to achieve a very low mechanical filter corner frequency (of around 3 Hz), and 3/8" ball bearing. $90 for a set of three, half the price of the Symposium Acoustics Roller Block Jr’s, and of a superior design.

I've just about accepted the fact that I'm getting two sets of three Seismic Pods to put under the Sound Anchor stands my ET LFT-8b's are on. About $800-$900, but justified by the degree of improved sound (or so I've been lead to believe ;-).

guitared, You should go under your house, find the area to support with bottle jacks and 4x4's. (or equivalent), After this, (if needed),you may look at look at other options above.

(All of this will keep you under your $100.00 target... I'll make a bet that it will be the ticket...just requires some effort.)
Be careful if there made of press board. Threads are a good idea (already mentioned). There are several good options. If you send me a PM I will show you something I came up with for ye old English towers. It widened stance and has them firmly planted. It made a sound improvement.

One option thats OK. If your going to go right into the cabs cut some 1/4" pieces to bond to the bottom, and uses spikes with wood screw type threads on opposing ends. IMO this is more durable than some of the other options. 
My post was out of what I feel to be the need to address the main issue first: your suspended wooden floors. (I have a similar issue) This should be done first and addressing this basic issue will be beneficial for every aspect of your listening pleasure and system upgrades from now on.
Good luck!
The main reason I like (spikes), it allows me to fine tune speaker placement which I find to be critical. If you don't feel a need to drain vibrations away, the Vibrapods may be one solution. Some may have monitors that allow this adjustment to be made between the speaker and the stand.

I've found that with suspended wooden floors, there will be (out of level) floor issues that require a more thoughtful approach that may include having several methods of adjustability.
Thanks again to everyone for the helpful responses.  

I just put two hardware store sourced jack-posts on top of cinder blocks in my crawlspace directly under the speakers, and wow what an improvement!  The low end has evened out through the lower frequency range where before there were certain bass notes that rang much louder than others.  I believe the level of detail in the mid and upper ranges is up significantly as well.  I read something recently about working to allow your current gear to sound as it was intended before investing money in new equipment.  It has been a fun learning process figuring out cheap DIY methods to get significant improvements out of the equipment I have been listening to for a few years now.  If my system wasn't in a family living space I would move on to acoustic treatments next, but I'll have to stick with creative use of pillows and blankets until we move to somewhere I can get a more dedicated listening area.  I already got shut down for the more "attractive" items by my better half haha.
A really cheap but effective method is to use hockey pucks instead of spikes but you would have no adjustments unless you add some threads.

That's not a bad idea, and I might actually be one of the few North Carolinians who actually has a few hockey pucks laying around...
"Drain Vibrations Away" is an obscure Dead song…note the "pods" under my speakers don't drain anything so much as they keep the vibration from the bases of the speakers cabinets out of the oak (technically turning vibration into heat, although it's got to be some tiny amount of heat). This could be rendered meaningless by the fact that both of my subs are on that same floor, but at least I know the subs are putting only vetted bass of 40hz and below…vetted…