Thank you, sir.  It's what I gig with.  I used to use an X3 bean with an Atomic 212.  Then I went for the HD100 with the 412, straight up.  Then I had an M13 and MXR EQ with that.  Now I'm compromising by integrating the X3 Pro into the rig.  I think it's the best option for me so far.  Perfect combination of amp models and effects.  The ONLY thing it could use is MIDI on the SV, but I find I'm really not missing it.  Especially when using that dual tone fading trick.  Now instead of doing a tap dance of potentially 2 to 3 switches, I just ease on back on the expression pedal and I get a smooth transition from dirty to clean.  Effects and all.  And, of course, custom EQ for each amp model.  Voila!

Based on what I understand,

You can do the trick with the X3L, just by turning off the effect loop manual on the control interface since the X3L doesn't have a physical switch like the pro.

One question:

How is the Strymon’s SVPre?

I have the same rig like yours, and considering installing the SVPre for my SV.

I mean how much is the difference with the SVPre installed,and the amp modellers from the XL3?

Coz I noticed that the XL3 is really good at duplicating the sounds, but it kinda fails to duplicate the organic dynamic/response of an all-tube amp lol

Okay, here's the deal.

The Atomic Reactor is probably the best tube amp solution out there for getting loud with an amp modeler.  Period.   Well, wait.  Let me qualify that statement.  An amp modeler with good live cab models.

The problem is the POD.  The Atomic speaker and cab is reasonably flat, but not necessarily full range.  It has guitar speaker and cab characteristics.  King and Kolbe were trying to strike a balance, and they did a decent job at that.  The original idea was to use the POD in Studio Direct mode with the Atomic.  What we users found there after much experimentation is that in comparison with some other output modes that sound GREAT through a PA is that they end up sounding compressed and unnatural by comparison on the Atomic.

So we tried Stack Poweramp where the mic models are disabled and the cabs are revoiced.  Sounds better than Studio Direct.  More natural, less compressed.  Breathes more.  Actually, sounds pretty darn good.  Until you put it in the room with a real tube amp.  Then you start to notice that there are still characteristics there that sound a little plastic-y.

Okay, so the natural progression here is to turn cab models off altogether.  Let the Atomic cab be the cab.  Whoa, big mistake.  The Atomic cab isn't a regular guitar cab, remember?  Flatter response, not quite full range.  Sounds like *** without a cab sim.  The problem here, I believe, is that the POD's revoiced cabs in the live output modes are revoiced to match standard open and closed back GUITAR cabs.  Because if you run it that way into a real guitar cab, it sounds much better.

So now you buy yourself a real guitar amp with a real cab and run your POD into the effects return in Stack or Combo Poweramp mode with the speakers engaged.  Sounds good.  But you just bought a real cab, and hopefully a good one.  Why are you trying to emulate something different?  So now you turn the cab model completely off and it sounds heavenly.  Like a real amp.  The Spider Valve by itself convinced me that Line6 nailed preamp modeling a long time ago...with the POD 2.0 !!!  POD X3 amp models, minus the cabs, sound just as great into this amp.  As they would into just about any real amp and cab.

Wait a second...what about my Atomic?  Is it all of the sudden no good?  No.  You've probably noticed that Atomic has joined forces with Fractal.  That's no accident.  Remember what I said about good cab models?  And remember how Atomic has just come out with an "FR" version of the amp with a tweeter and redesigned cab?  They are going after the Fractal community's need for a good amplification solution, and from what I've heard, it works.  What I'd like to hear is how one of these FR amps does with a POD in Studio Direct.  I bet it does well.  Unfortunately, they aren't shipping yet.  And they only have a 112.  Not an option for me.  I need a 412.

To answer your question about how the power sections compare...I'm not quite sure.  The Atomic power section is designed to be as close to a linear amplifier as is possible.  Meaning that you're not going to get power tube distortion from it, at least not in any meaningful amount like you would when you crank, say, a Marshall's power section.  You'll still get a subtle compression from it owing to the tube amp's nature.  The only reason the Atomic is powered by a tube amp is because it turns out that's the least expensive way to make a high quality, near linear amplifier.  With today's tech, it could be done with solid state, but it would cost a fortune by comparison.

Now, I'm not 100% certain on this, but I don't know that the Spider Valve power section is much different.  Cranking loud on the clean models, do you hear any power tube distortion early?  I guess a little, but not a lot.  I do hear a bit, or at least I think I do, with some of the other models.  But it's a 6L6 based amp and they break up differently than an EL34 would.

Anyway, sorry for the encyclopedia article.  But I think this is the first time I've gone on the record with this.  Hope it made sense.

Edit: as an experiment, try this.

Run a regular 412 cab of your choice as an extension cab on the Atomic.  Turn off the Atomic internal speaker.  Turn off cab models on the POD.  What do you hear?  Sound good?

I'll create something over in the "share your tones" section later tonight.  Have to dump my bundle anyway.