Thick vs. Thin electric guitar sound
I'm going into dangerous territory tackling this topic...
...because it's completely subjective. But I'm tackling it anyway.
The argument I've seen most for what defines thick vs. thin guitar tone is when Eric Clapton in 1969 switched over to the Fender Stratocaster from Gibson guitars. Gone was the "thick" sound of the Gibson, replaced by the "thin" sound of the Strat.
The specific thick Clapton sound that's being referred to can be heard in this film clip from 1968 where at the time Eric was using an SG - and note that even when he describes a certain sound as "thin", the sound is still thick with plenty of midrange bark:
It is the pronounced midrange of the humbucker that really thickens up the sound more than anything else. And you don't need an SG to get it. The Ibanez RGA42FM seen at top can get the sound just as well, or pretty much any solid body electric with humbuckers.
Traditional Strats and Telecasters with single-coil pickups don't have a thick sound, nor are they meant to.
What single-coils are best at doing is clean tone. When you want the absolute best clean electric guitar sound, singles are required. There's just no arguing that point. When clean is the goal, get the singles.
For lead tones however, you may be in for a fight trying to get the lead sound you want with single-coil pickups.
Back in the 1970s when there was a lot of electric guitar experimentation happening, there were many attempts to make guitars that had both the crisp clean tones of the single and midrange-heavy lead tones...
...hence why the '72 Telecaster Custom came into existence:
Single in the back, humbucker in the front.
Best of both words, right?
Sometimes you want a humbucker in the back and a single in the front, and you can't get that out of a Tele Custom.
What about going with H/S? That brings about the problem of a loud rear pickup with buckets of midrange and a quiet trebly pickup in the front. And even if you have 4 controls (two volume, two tone), you either have to turn one pickup waaaaay down or the other waaaaay up just so they're not fighting each other constantly. Kind of annoying to deal with.
What about an HH guitar with coil-split humbuckers? That would work, right?
No, that doesn't exactly work either. Or rather, it works, but a common complaint by those who use humbuckers that split to single-coil is that a) the volume drops too drastically when split, and b) the split-to-single tone doesn't sound "single enough".
Yes, there are pickups that when split do retain the same volume as they do in full humbucker, but the same tonal problems remain. The humbucker doesn't have enough midrange bark, and the split single doesn't have enough top end treble bite.
If you want thick with midrange, go humbucker. If you want thin with treble, go single.
Many have tried to build the all-in-one guitar that has both the thick and thin tones. Heck, I even owned one myself at one point where I had a Fender Stratocaster HSS with a humbucker that split to single-coil.
In my experience, both thin and thick guitar tones can't be had in the same guitar. This is one of those instances where if you want it all, you have to own 2 guitars to get it. One loaded with singles, one with humbuckers.
The simple solution to have it all is owning both a Strat and a Les Paul, or a Telecaster and an SG, or a Jaguar and a Gretsch Double Jet... etc., etc.. Again, one with singles, the other humbuckers.
Is it possible to "thicken" or "thin out" an existing pickup?
With singles, the one I know to have the most "thickness" to it is the P90. The P90 brings the growl because it has more midrange response to it. Of course, the tradeoff with the P90 is that you lose some of that top end treble compared to a Strat or Tele single-coil pickup, but again, it does bring the midrange thunder when overdriven.
But where Strats and Teles are concerned, those are all voiced thin very much on purpose - which is how they are supposed to sound because that's where you get that great top end treble response from.
With humbuckers, the treble response of a single isn't there and instead pronounced midrange. And yes, that's how they are supposed to sound. That midrange is great for lead tones, "fuller" sounding jazz chords and so on.
Yes, there are some humbuckers that are voiced to have more treble than others, like the kind found in many Gretsch guitars. But still, a Gretsch won't be as trebly as a Telecaster bridge pickup - nor would you want it to be.
EQ doesn't fix everything
Using the Strat as an example, okay, so you don't have a lot of midrange response coming out of the pickups, so you just add in EQ at the amp and problem solved, right?
Well, if you have ever tried that, you know that doesn't magically change a Strat single to a humbucker sound.
But let's go two steps beyond that. You add in compression (this "squashes out" some treble and raises midrange some), and punch up the midrange EQ, and wire your Strat so that you have tone control on the bridge pickup (traditional Strat wiring does not have bridge tone control), allowing you to take out that biting top end treble by rolling the control down to around 5.
Does this work? Yes, you are better off than before with thickening up the Strat tone, but at the end of it all it's still not going to sound like a humbucker. At best you have a P90-ish sounding pickup at that point. That's not a bad thing, but still not a humbucker.
On the humbucker side of things, scooping out the midrange and punching up the treble control at the amp won't magically get Strat tone out of a humbucker either. You're just not going to get Strat-like treble bite out of a humbucker no matter what.
What's the all-in-one solution?
There is none. Nobody has ever been able to get both the thin and thick sounds in the same instrument. Many have tried.
In fact, I will give you a real example of a guitar that attempts to do it all that you can buy right now at the time I write this:
This, dear reader, is a Schecter Ultra III.
You may look at this and say, "Three humbuckers? I've seen that before." Yes, but you haven't seen it where every pickup can be split to single coil. Look at those 3 switches below the pickups. Those are split toggle controls for each pickup.
On paper, the Ultra III sounds like it can do anything. Every possible pickup sound you ever wanted out of one axe. Perfect guitar, right?
No, it's not perfect. It still has the same issues any guitar has with humbucker pickups that can be split.
So even when a guitar is built with all the control you'd ever want, in the end you would still have been better off just buying an all-single guitar and a separate all-humbucker guitar.
This is not to say the Ultra III isn't good. It's great. Very cool guitar. But it won't replace a Strat or a Les Paul.