Well i bought a single DB840 sub to try out this swap.
"only bought one in case this was a bust and did not improve the sound/work out."
Important thing to note if you plan to do this at any time you must remove the rubber seal around the DB840 in order for it to fit in the pre fabbed spot for the 8"driver. If you do not it will not fit.
This of course means you need to make your own gasket so get some mortite ready.
I have not done that step yet but will tomorrow.
Also if you plan to do this get new screws the old ones sheer off really easy trying to get the new sub to seal all the way. They are easy to snap like a twig trust me.
On to the speaker/seal, i currently do not have a seal to finish up the job. I will do this tomorrow but i had to listen to what the new driver had to offer even if it didn't have a true perfect seal. It isn't horrid but you can feel a little air leaking out at the moment so i will correct this once i get to the hardware store and get the good stuff.
Overall with the new driver the 15's can handle more volume/power instantly. Most likely due to the power handling of the new unit.
To me the stock unit did not go deep enough was kinda slow about things overall sloppy in general and i had to have a sub going to make me happy with them.
Note that my 15's have upgraded crossovers done by Ben with sonicaps already as well and this is how i feel about the stock bass driver even still.
If pushed to hard the stock sub would reach it's limit bottom out sometimes and always kinda had a resonant sound to it. Just not good.
The DB840 has a vast improvement in bass reproduction. It is quite a bit faster plays much deeper without going out of control no longer has the same resonant sound the old driver seemed to produce and cleaned up the sloppy sound greatly!!
I am only running one atm because like i said i didn't want two of them in case it did not work out but, even with the single sub i can tell i will not be running a sub for 2 channel any longer.
With the new driver sitting in my chair i could feel the bass beating in my chest and feel the vibrations thru the floor in my feet as if a sub was running.
With two i would imagine the output would be very nice and " I placed an order for the second one before making this thread."
If you feel your 8" driver is your main displeasure in your LSI15's i suggest you try swapping them out with the DB840 single voice coil 4 ohm subs.
The difference is greatly noticed and a massive improvement to my ears.
The subs do not cost alot of money and they will tighten up the bass big time.
Using a test CD to sample bass notes for a hz vs hz comparison stock vs DB sub.
The DB sub was able to reproduce very livable bass down to 40hz and really comes alive 45hz and up with some intense spl for a 8inch driver.
The stock unit was able to reproduce livable bass down to 53hz and had average spl output at around 58hz.
This is of course being nice about livable for the stock unit as it falls short all over the board vs the DB sub. Also note the spl output for the stock is down across the entire board of HZ ranges and it is very noticeable for the entirety of a 120-20hz drop.
Doro told me about this swap so a BIG THANKS goes out to Doro for filling me in on this cost effective very worth while mod!!!!!