I just received my Chiefton Low D Custom today and it seems to be very quiet in the low octave. Is this normal or is it me. I am a new comer to the low whistle and have been using a Susato Kildare which is nice and loud. I don’t mind the quiet factor but I want to make sure I am not “doing it wrong”.
The range of expression on a low whistle is larger than that of a high whistle. I’ve found that changing the shape of your mouth, throat and position of your tongue can produce a louder or quieter sound in the low octave. Of course, that is within the bounds of the whistle’s volume range. I find that my lower octave improves over time as I play the whistle more.
Apparently this quiet is simply a characteristic of the solid brass body. The alloy body is not nearly as soft in tone or volume.
It’s possible that Phil voices brass and alloy whistles differently, though I have no idea if that’s the case. But the tone and loudness of your instrument has almost nothing to do with the material the body is made of. It has everything to do with the specific geometry of the fipple (windway size, window size, blade position, etc.), the bore diameter, and the size and layout of the fingerholes.
Oh, and it’s Chiftain. 
Not that he needs my help but MTGuru is right on regarding the material having little to do with tone or volume. I have several OS brass Chieftains including low D, low F and low G and no one in their right mind would consider these quiet whistles. Like every other Chieftain I’ve seen they are assault weapons, loud and heavy.
You might also test to see if you’re pushing it enough. As noted but the Madman, my low whistles, including the Chieftains, will play both softly, though flat, and loudly in order to bring it up to proper pitch.
If it’s quiet in the lower range I suspect it is intentional voicing but that would be an odd change since IMHO one generally wants a strong bell note. I’m sure Phil would settle the matter. Why not ask him?
ecohawk
I am chalking it up to my lack of experience. This whistle uses the same mouthpiece for the brass and alloy bodies so the voice should be the same. I am on vacation for the week and will probably live with this whistle in my hand.
Oh, and it’s Chiftain.
Chieftain even
Oops, I must have been thinking of Chifftain. 
There are other things that could cause a difference in volume between the two bodies, among the one’s I’d suspect first:
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The bore diameter of the brass tubed body could be smaller than that of the louder body. This can be more common than you’d imagine for a variety of reasons.
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A small leak at the tuning slide if the fit isn’t just right with the brass body.
Loren
You really have to warm up the brass tube for it to sound right. Also, check to see that you have it positioned at the right tunning position. I know that with the Kerry Pro sets the tubes are machined differently (probably, as Loren pointed out) to adjust for any difference in tube thickness.
I had a Chieftain once and it was very quiet.
One thing though, if it’s tuneable make sure you haven’t got the slide too far in and are under-blowing. Give it a wee bit of welly! It’s very common to see folk new to low whistle (or even high tuneable ones) to have the slide too far in, usually thinking it’s flat in the normal position. It’s just flat because it’s being tickled. However, I haven’t played a chieftain for years. The old low ones were too quiet and the high ones far too loud in my opinion.
edit: Sorry, missed that tuning was mentioned in the post above.