HOB on a salt setup?

Not at all. My first two salt tanks had hob filters on them. They work fine.

Give me some specs on your aquarium...what you are trying to do with it (reef, FOWLR...ect)...everything you can.

There is nothing wrong with a HOB filter, but they might not be my first recommendation, with a large/powerful enough skimmer you probably wouldnt even need to run it.

Be careful with that DSB...I have had bad experiences with people who think that a DSB is the magical-cure-all for water quality issues. It isnt. And cleaning the substrate in closed systems will always be necessary in my mind.

That being said, I currently run a heavily stocked (blue hippo tang, midas blenny, 2x clarkii clown) 75g reef, and it is doing great (ammonia, nitrite, nitrate are all zero with bi-weekly h2o changes) and is housing mixed corals but mainly sps, with only a AquaC Remora skimmer. You can buy prefilter boxes for these that would also give you mechanical, biological, and chemical filtration, but I dont think you even need that. I would skip the HOB and get yourself a high quality skimmer, and an overflow box if you felt it necessary.

Also 150# of LR in a 75 is a lot...I had 140# in mine and the rock ran from both sides at one height to within 4" of the surface...it was good for the grouper that was in there, but not much else. Remember, you have to have places to put corals.