On the long wall or short wall- Speaker set-up

I’m fortunate enough to have a dedicated room built over a garage with dimensions of 16.5’ W X 27’ L. 
Room will be vaulted with the long wall having the sloped ceiling and the short wall having the gable. 
Loudspeakers will be the PAP Quintets where I understand room treatment may be less extensive than required with box speakers. Is there a means to predict or measure the preferred set up or possibly left to the acoustic professionals. Asking for your personal experiences. Thanks all. 
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You will hear people tell you the side wall is definitely the best. You will hear people tell you that is crazy only the end wall in that room. For sure a whole bunch will tell you only spending as much money on professional acoustic consultants as your system, and maybe more, will get you there. 

Been there. Done all that. Yes including paying and seeing what and how the pro's do it. 

Save your money. Nobody knows. For the simple reason nobody has your ears. Only YOU know your personal preferences. And if you spent the money to build the room I sure hope you have your own reasons.

So what you do, try both ways. There's no getting around this anyway. The whole idea of speaker/listener placement is finding the best balance among frequency response, imaging, and convenience. All matter but only you know how much weight to give each of those. 

This doesn't need to be that hard either. The testing I'm talking about you could sit the gear down on the floor and try the speakers each way. Doesn't have to be pretty. Doesn't have to be precise. Not at this stage anyway.

Try several positions. Listen primarily for smooth bass response, but also for spaciousness. Really good imaging requires really precise speaker symmetry, which takes time and is not what you're after at least not initially. So notice, but do not focus too much on it. Do the same both side and end walls.

Without a doubt you will find you prefer one over the other. That's the one you go with. 

Keep in mind this is not the end but just the very beginning. A ton of tweaking lies ahead. But this is the process I went through, and it has been shown to work really well. It don't cost nothing but time, and you learn a whole lot more this way than anything else. Nobody ever learned a thing swiping a credit card. So go and try and listen. You will see.
I recommend you experiment, if you can, with an angled/diagonal placement,  that's neither parallel to either wall.  You can mitigate a lot of room modes in this way.  
To all, perfect advice, thank you. My intention was to allow the architect to design a couple of windows and the electrician to place the dedicated circuits in advance. I have to go before the town board for a variance that I will most assuredly receive but they ask for details. I think I have the advice I needed from an experienced community. Will report in as the project advances. 
Avoid windows if you can. Even closed they're horribly noisy. My back yard is a forest. Even so the noise coming right through the window panes tool 1/2" thick MDF panels to shut out. 

Avoid architects if you can. They charge a fortune, and love complicated expensive designs that they use to impress and distract from the fact any competent designer could do better for a fraction of the cost. Which I know, having taken competing bids from a number of them for my room. In my case the best design was also the cheapest of the six we got.

Avoid dedicated circuits, plural. You want one. One. Read the above again. If that doesn't do it, listen to Michael Fremer at 15:50

As one datum point:

The dimensions of the Great Hall (Golden Hall) of the Musikverein in Vienna (the home of the Vienna Philaharmonic) are: 161’ long, by 62’ wide, by 59’ high. The Hall probably has the best acoustics in the world for symphonic music.

You could scale down the dimensions for your use, although the width vs height could be a problem.

Over the years, I’ve heard over 20 performances in the Great Hall by the VPO, and that’s what I use as a reference for my own system. Obviously, I’ll never get there, but it helps.