The dampening coefficient that you get will be a result of the sum of the components, according to their share of the total stiffness, and energy stored in said stiffness.
For example: an 8 inch diameter pipe, 3/4th inch thick side wall has 226 inches^4
a solid 8 inch rod is 402inches^4
The stiffness of epoxy granite being on the order of 1/8th (maximum) that of steel, means that if you fill the .75" thick sidewall, 8 inch outside diameter steel pipe, you've only added 1/8th of the difference (402-226)/8 is 22in^4 equivalent. this means the epoxy granite will take 22/(226+22) parts of the strain, or about 9%
Assuming the steel has a dampening coefficient of .001, and the epoxy granite a dampening coefficient of .05, this means you now have a final result of (.91*.001)+(.09*.05) which is .00541.
.0054 is a lot better than .001, (and this is enough to make people think its no longer resonant) but the weight was nearly doubled by adding the epoxy granite, thus the resonance frequency dropped by the square root of 2 if i recall correctly, and the final dampening coefficient still worse than cast iron.
often times very high dampening coefficient, but low stiffness materials such as sand, rubber, silicone, polyurethane (density can vary by two orders of magnitude for polyurethane) are added between tuning forks made of very stiff materials. they call this "constrained layer dampening" and the purpose of it is to cancel out specific frequencies.
for general machine tool purposes, the best material remains cast iron because you can machine it cheaply and it is stable after heat treatment.
if you want the highest dampening coefficient for the highest stiffness, solid granite may be the best option, basically epoxy slabs of cheaply available granite counter top together (ideally, get all the pieces cut from the same sheet) and bolt or epoxy on certified flat granite straight edges (or cut up a surface plate) with inserts drilled and tapped for the linear rails.
there are very few other materials besides granite and cast iron that have any use in machine tool bases, you may be able to get higher dampening coefficients from epoxy granite as opposed to solid granite, but such results depend entirely on how well you fitted the mass fraction of all the separate ingredients of the epoxy granite. (too much epoxy will drastically reduce the stiffness)