I've played with this a lot,using weight tubes,aquarium tubing,all types of nylon and poly chords and powdered compounds like borax.
Starting with a well tuned arrow,they all made the arrows quite a bit stiffer and murder to retune.You may use a flexible,full length inner weight but spine is still changed,drastically.
This is why:
1.Adding weight to the rear of the shaft stiffens dynamic spine.
2.All of our arrows have a balance point somewhere forward of center.
3.This means the rear of the arrow,behind the balance point, is longer(longer steering arm)and x amount of weight in the rear has more effect than the exact same amount of weight applied to the front.
4.When we install a full length weight tube,there is a disproportionate amount of weight behind the balance point.It depends on the original balance point but the weight tube puts maybe 2/3 of the weight behind the balance point and 1/3 forward of it.That's exactly the opposite of how it needs to be IF you are staring with a well tuned arrow and want it to stay tuned.
5.If that arrow,was truly tuned to start,and you change that balance point,you changed tune.All full length weights change the balance point quite a bit.
You can test this very quickly without permanently altering your arrow.Find something to balance your arrow on and clearly mark that balance point.Now stick some weight to the front,like split shot and tape,chewing gum-anything.Watch how the balance point shifts.Move the same weight to the rear and see how far the balance point shifts,as opposed to when it was upfront.It will have moved farther.
Again,I'm only talking about an already tuned arrow,then adding overall weight.What I've learned from a lot of testing,if my arrow WAS tuned and I shifted that balance point(same shaft length),it was no longer in tune.
What I did finally discover was that I could increase the overall arrow weight as much as I wanted,by adding some of the weight to the front and a smaller portion of it to the rear.It can't be equal.Say I wanted 100 grs more overall arrow weight.Something like 65 grs or more would have to go up front and the remaining 35 or less goes to the rear.That percentage depends on what proportion of the shaft is behind the balance point and how much is forward.
I found that I could quickly test how much I needed front and back by adding the stick on weights till my balance point.Then it was a matter of figuring how to build that much weight in the front and rear of the arrow.
Now,if you want to add a weight tube before tuning and tune from their,you can but be prepared to have to add more up front weight than you imagined,and there will be no way to predict the final arrow weight.It will be what it will be.
I found that adding the weight front and back,I could make the arrow exactly the finished weight I wanted.Sorry for the long post but I blew a lot of time figuring this out and maybe someone can use something there.It seemed complicated getting there but once I figured out the formula,it is very simple now.
From what I've seen,if I have a well tuned arrow to start,changing it's balance point,changes tune.It's that simple.