I have no idea what is in your range cubes or what nutrients are in your grass/hay(or whatever).So no I cannot tell you if your range cubes are raising your TDN. I don't even know what part of the country you are from to give me even a shot at guessing what you have to work with as far as forage..
Not having the basic facts in your post, lead me to your best solution-- which was an exchange of facts and options with your extension agent.
I can tell you that the first thing you need to do- is have some kind of idea what nutrition is in your forage
Then balance the protein-- if theres not enough of that cattle cannot digest forage. Next is to get the energy level up to a workable level.Without adding so much starch that it interferes with the rumen bugs.
If all you need is alot of protein- distillers might be the best supplement
If you need just a little protein a lick tank might be best.
If you need protein and a little energy a high protein pellet might work better- especially if you need to stretch forage
If all you need is energy a lower protein pellet may be what you need
If you have all the necessary nutrients in your forage- you may need nothing but some cheap bait pellet.
How much is your time worth? Would extra labor have more value over spending more money?
would it be better to feed a pellet with minerals already in it over finding another way to supply minerals?
Do you have storage for distillers? Are you using enough to be worth storing it? (the way things are going I wouldn't count on having a get it as you need it supply)
I have no idea what your goal is-- are you trying to stretch hay(or whatever) or just supplement?
are we talking cows, calves, heifers, cows with calves?
Then you have to add in what would be the cheapest way to go-- and you gave NOTHing that would help there.