FWIW,
Using GT Trad 3555s
I'm experimenting with 3/16 wood dowels as "weight tubes." I find they are just larger than the GT nock inside dia. I trim them (point taper so to speak) and press in the wood dowel for a snug fit to the nock then trim the dowel so the other end contacts the tip insert back. I take scotch tape at four points along the shaft to make the dowel stable along the length of the shaft (bushings, if you will - also keeps down rattling noise) and it makes the spine stiffer. Kinda like turning a 3555 into a 5575. The only problem is weight consistency with the wooden dowels. With the dowel pressed into the nock and touching the insert back it doesn't separate with the inertia of release or upon impact.
Very experimental at this stage for me. Wish I had a spine tester but flight tells me it's stiffer thus allowing for heavier points and a better/higher GPP arrow.
Arrow with no point before tube - 301
Dowel weight w/tape - 139
440 total
with point 616gr (176 gr Eclipse single bevel)
I suspect moisture could cause weight variances so sealing the wood is another consideration.
Just passing this along in case someone else wants to try it. I'm not sure if I'm duplicating an idea someone else has had but thought I'd throw it out there.