Acousta-stuf UK?

Acousta-stuf is a nylon-polyamide material. It's certainly very good, but frankly, no more or less in this application than plenty of other available materials, most of which are a whole lot cheaper. Remember, all damping in an MLTL is there for is to attenuate unwanted harmonic resonances, and a properly designed enclosure shouldn't need much. All materials have different attenuation characteristics, but this tends to be of greater significance in ~aperiodic TLs, rather than highly resonant QW lines like an MLTL.
Hi keladrin

have you thought about long haired wool, the original material(Used to be called Dr Baileys wool). I have listened and measured long and hard to both approaches and prefer LHW by a good mile. If you are interested drop me a PM and I will look out the addresses for suppliers, or depending how much you need I think I have about 2kgs left in my stash....This stuff is now hard to come by due to EU regulations having changed......

I can't find the reference at the moment, but there is also the question of arresting the wave speed to achieve phase coherence front and back, which I understand is what LHW is best at.

Not heard that notion before. Sounds like one of Bradbury's ideas though, or something based on it. I can point out, on a related matter, that damping does not slow the speed of sound sufficiently to have an especially dramatic affect on the tuning frequency. And ideally, you don't want it to do anything at all to the fundamental in a properly designed resonant line. AFAIK, LHW's damping functions are, weight for weight, probably the best out there -perfect for an aperiodic line like Bailey's cabinets. But the material shouldn't be a particularly significant issue for QW lines, because they don't need a great deal to damp out the unwanted upper harmonics, which are pretty easy to take down.
Hi Scottmoose,

I always fine-tune my cabinets after damping and it does always effect the tuning about 3Hz or so for half fill. You can see the effect of adding the wadding by using a frequency generator and marking the point at which cone excusion is minimum. This is easy to see with a Jordan as you get a definate 'width' of the blur when viewed sideways as it vibrates - just shine a light on it.
Oh, I'd be the last to disagree, but as I mentioned above, my point is simply that I don't consider 2-3Hz difference in F3 to be especially significant. 5 - 10Hz? Sure.

Either way, IMO, it's better to design a pipe to go to a target frequency from scratch, rather than trying to achieve it with damping materials. If you get another whisker of extension once it's damped down, that's simply a bonus & secondary to the damping's primary objective of flattening the harmonic resonances.