So, I think there’s a couple important things in this thread.
Firstly, I think the question of the correct way to practice is really important in how players develop and I wish I was aware of the research being done on this a long time ago back when I had all the time in the world to practice.
Anyways, I think the idea of practicing a certain way and excluding any other forms of practice is something that should be done with caution. Certainly, if there is research to show that a particular kind of practice has no benefit then it should be avoided but my instinct is that most methods of practice have some benefit. Furthermore, I think the idea of varying how you’re practicing is very important. My understanding is that research into the area shows that varying how you are practicing improves results. I think the canonical example is having basketball players practice only from the free-throw line vs. having them practice from all over the court. If I recall correctly, the latter actually resulted in better performance from the free-throw line when tested. In the case of guitar, practing unplugged, plugged in but with a clean signal, plugged in with gain, and whatever other variations you want to throw in gives you better insight and develops subconscious abilities to adjust your playing as needed, basically making your more flexible in the moment as a player.
Secondly, I think it was Andy Wood (someone please correct me if I’m wrong here) that was talking about how sometimes it’s important to push yourself and accept a certain amount of slop early on in the playing to work out the mechanics of a particular passage. Once you’ve got the mechanics understood and are able to play it at the desired speed, albeit it with some level of imprecision, then you start focusing in and refining it. I think this is counter to the idea of playing slowly and trying to always play perfectly. Sometimes, it seems like there’s a benefit to pushing yourself, in this case playing without an amp, to get 90% of the way to where you want to be and then refining it in a more practical way, i.e. with an amp/gain/etc…
Sorry, for the long post. This one piqued my interest.