I played a rosewood Low D combination set he made many years ago, basically one of his standard Irish flutes (which I really like) with a Low D whistle head also.
With the flute head it played like his rosewood Irish flutes usually play, big bellnote etc, but with the whistle top the low notes were too wimpy for me. I don’t know why it played so differently with the different heads, but then again I know nothing about whistle making.
That was a long time ago. Presumably he got the Low D whistle head design dialed in later.
I guess it might have to do with the flute head stopper position. It has a big influence on the bell note. Moving it further out will strengthen the bell note (but might weaken other notes). But the further out it sits, the more you’d either have to increase the hole size or the taper of the flute to get the octaves in tune. So more taper can still mean “fatter” sound on a flute. But that will reduce the volume and “oomph” of the instrument when used as a whistle. For me a whistle/flute combo seems like too much of a compromise. Might work on higher keys like A or G though.
It might be possible that there are tapered low Ds out there with a fat sound though. Maybe when constructing the mouthpiece accordingly. My loudest low D (not tapered however) is the Qwistle, which has a tapered windway and very high backpressure. I wonder how such a mouthpiece would sound on a tapered bore body.
I read a few comments on the “organ pipe” sound of the Howard Low D and I’d like to inform forum members that now Howard Music offers an alternative version of the mouthpiece, that delivers a less reedy and more Breathy/chiffy sound. I have it and it is fantastic.
Here is a YouTube review of the new Howard low D where she plays using all three heads. https://www.youtube.com/watch?v=NCZaV4_deEU It is done by CutiePie. It helped me decide which head I liked best.
.