I suspect that Steven Shand is reading this thread and simultaneously banging his head against the wall while restraining himself from posting.
When I have had a custom steel frame built, I have never told the builder what angles I want. I have told them what I was going to use the bike for and what I was looking for from the bike in terms of feel and handling, and relied on their skill and expertise to choose tubes and geometry etc. to deliver that. I have not been disappointed: my steel bikes are my favourites over and above high end off-the-shelf carbon and titanium, and I have no idea what their angles are – I just ride them.
With regard to the off-the-shelf Stoater’s slack head angle, this angle is part of a complete design package, which was doubtless arrived at as optimal for that particular frame size for >90% of riders. As well as for handling appropriate for the bike and type of riding/terrain for which it was developed (in conjunction with the rest of the geometry), the head angle has probably been chosen to maximise mudguard and tyre clearance (more important given that the Stoater will likely be used with large tyres and/or off road where low speed manoeuvres would be more likely to cause toe strikes).
That Shand as a very nice-but-dull 70-71 degree head angle. OF course you can ride it on the road, but… it will handle like the touring bike it is designed to be. For a proper road experience you really want something with sharper handling. Look for 72.5 or 73 degree head angle. A 73 degree parallel design is an excellent handling road bike. You should be able to fit a rack if needed.
Even for Audax riding, I would go for something sharper. I do, however love the flex of steel.
Colnagos have head angles as low as 70-71 degrees, and that does not appear to have impeded their success commercially and in races. Their handling is often described as very stable and neutral, especially at high speed, qualities which may also be desirable in an audax bike.
Morecashthandash, you have already specified your design brief, and the Stoater does sound a very close fit to your requirements. However, the advantage of going to a good quality UK frame builder like Shand, is that you can discuss what you want from the frame and bike with him, and if necessary/appropriate you can go for a custom build.
Just got the feeling that my heart may be ruling my head, there are cheaper options out there.
The right bike, and the one that is best value, is the one that you makes you want to ride, and you are probably more likely to be confident of getting that from the likes of Shand.