Re: walnut & pickups ?
In my experience, Walnut will have somewhat of a notching effect on some of the midrange frequencies. It's a very complex wood, and somewhat compressed. So it will comb out certain frequency blocks while limiting any others from being really pronounced. So if you use a pickup that's heavy in the midrange, the output will be heavy in the midrange, but only to the degree of the pickup's content. In other words, you won't have the compounding effect of the overlapping midrange frequencies naturally inherent in the guitar.
And even now I'm exaggerating the situation a little to make the point. Of course changing pickups in a Walnut guitar makes a difference. Actually I think it makes a difference in exactly the opposite way mentioned. It doesn't make everything sound like Walnut, rather, it makes every pickup change represent the actual variation in the pickup itself.
Let's illustrate to the point of obsurdity to clarify: Say you have an Ash bolt-on that could be represented on a "tone bar" as 5/7/7 (I just made those numbers up) so then you add a Hot Rails bridge pickup at 5/7/4 and instead of adding them to get 10/14/11 you actually should be multiplying them to get 25/49/21. Does that make sense? Think of it more like the guitar is in "series" with the pickups, not in parallel. You don't get the sum of the parts, it's compounded. So if, in contrast, Walnut produces a 5/5/5, then the dramatic effect of the pickup change is lessened. Now, if you looked within the 5/5/5 there would be a lot of little ups and downs, too, on narrow frequency bands. So there is a good deal of Walnut character coming through the pickups, but you can use a wider variety of pickups successfully. That's good news and bad news. It means you can have your pick of pickups and a vast majority will sound great, but it's a little harder to really find a "sweet spot" where a certain pickup magically bonds with a guitar's natural tone.
So while I'm not defending the obsurd, and I'm clearly making a mountain out of a molehill to illustrate where someone could come up with that logic, there's some truth to it at the end of the cable.
