3 years, one month, 6 days, 2 hours and 12 minutes... please use the search function or Google the
next time instead of bumping a really old thread.
And to answer your questions a bit:
Just think of IRs, also known as Impulses or Impulse Responses as a full parametric equalizer with
an incredible amount of bands. IRs are not dynamic, due to that it only sounds similar to a cab and
not 100% the same (still some of them are really close).
This is how a IR is created in a really simple way:
You send a signal through your amp, into your cabinet and record this with a microphone.
You send the same signal just through the preamp of your amp (sometimes poweramp, too) and record it, too.
Now you use a so-called convolution program that does something like this:
(Amp+Cabinet Signal) - (Amp Signal) = IR
This will be saved as a .wav file, due to different sounds created by different microphones, positions
and cabinets, there are loads of different sounding IRs.
In your case (using LePou Plugins) you want to simulate a normal guitar rig virtually, so your chain
will probably look like this (just a basic rig):
Guitar-->Interface-->Logic-->LePou Lecto-->LeCab
Lecto simulates an amp (in this case a Mesa Boogie Dual Rectifier) and LeCab simulates a cabinet.
Like I said before, the different IRs (in your case 15) will probably sound different because they
simulate different mic-positions, different cabinets or whatever.
If this doesn't answer your questions, please consult Google or pray to the almighty Spaghetti Monster for enlightment.