Heat-Lock is definitely the best way to go and the easiest. It works with both raw and paper packed veneer. I used to use paper backed veneer but now use raw veneer.
It is really easy to find beautiful pieces of raw veneer to fit whatever speaker you are building. And you get to choose your veneer instead of getting a mystery
sheet of paper backed veneer. Raw veneer is also thicker so it easier to work with.
Contact cements are not what they used to be. Most of the water based ones are terrible. Titan DX is an exception and is available from Veneer Supplies too.
DAP contact cement in the red can is pretty good if you can still find it. Stay away from the blue and green cans of DAP.
Iron on with Titebond II works but has a high failure rate 2-3 years down the road so that is up to you if you want to take the chance.
I really don't see any reason to use anything else than Heat-Lock unless you can't get it. It is just too easy to work with.
I've never used Heatlock so can't speak to it's actual use, but I'd guess it's some form of PVA glue...like Titebond.
I'd be curious to know more about the alleged failures of Titebond II though. In my experience glue rarely, if ever, fails. It's far more likely an installation or substrate problem. I remembered something read long ago about using it for iron on veneering, so I pulled out a couple of books and boned up some.
White glue has a relatively low "reactivation" temperature...180°. Turns out its actually the easiest to use because of that. Veneer is not a structural element so killer strength just isn't required. Red label Titebond re-liquifies at about 250° so you've gotta get your iron hotter to make it work. Because of it's formulation, Titebond II requires about 350°. I don't know if household irons even get that hot. Given that, I'd guess that the "failure" was actually never bonded properly in the first place.
We humans have a nasty habit of assuming if something is better for one application it must be better for another. Not the case for wood glues and veneers. What would be the need for a water resistant bond on a speaker? You'd have a whole bunch of other problems if it ever got wet enough to re-liquefy even white glue.
When it comes to contact glue, I've tried many, but have a head and shoulders favorite and that's 3M 30NF. If it's got a downside it's that it takes longer to flash than solvent based glues. Well, that and it's about $100 a gallon! I generally wouldn't contact glue with veneer but it has it's place.
