PERFECT 10w x 18d x 9h room, any good?

Exactly ratios are just math... Fact is space is going to be needed regardless in order to create a stereo acoustic environment... I mean we could say a 3ft wide hallway 10 feet long and 6 feet high is a perfect ratio in a listening environment.. Obviously for bose cube speakers maybe and somebody thats about 4 foot tall!!

I am just kidding, but its gonna be a challenge in this room, you will most likely save some time and money doing some sort of reasonable acoustic treatments, probably consisting of some diffusion and absorbsion product, otherwise you might end up with a tunnel effect, or be listening at such a huge toe in and so close that you might as well put on headphones :-)

You might want to start with just some stacked GIK Tri traps, which at about 300 bucks a pair do well in smaller rooms with helping soundstage and bass response placed in the corner behind speakers.

I would call them and ask how they handle really small studios and like control rooms for monitoring recordings etc...

In the end all you can do is give it a shot for free, you own the room.. Tweak around and see what happens.

Good luck

Concrete walls? Concrete floor? What kind of ceiling?
Any windows? Stairs/stairwell or any attached spaces...thin doors to laundry or other service area?
All play a part. but you start with the basic dimensions. If concrete walls, best of luck and a substantial budget for accoustic treatments.

Undertow, there is a reason for the phi ratio. This is much more than just 'math', which I find most people irrationally shy away from. I am not going to give a history / math lesson, tempting as it might be, but the phi ratio is one of those 'perfect' numbers and in addition to some interesting properties, was also known to the ancients who prized it.
But, in short, for audio purposes, a phi shaped room, or speaker enclosure for that matter has few or no standing waves. Or, more exactly, a frequency which is an even wavelength into more than 1 dimension. Again, multiple bounces count...like any 2 walls across from each other. or any 3 walls, kind of imagine a ball bouncing around. You'll get the picture! Dead give aways for potential problems are rooms with even multiples of dimensions, like the OP's 9 and 18 foot dimensions. The worst room? probably some kind of cubic shape.

Magfan
I understand, not sure you see what I was actually saying.. Too small is going to be too small an acoustic space regardless of a Phi ratio. I agree there is definitely an optimal ratio I am sure.. Point being if your forced into a room barely as wide as my bathroom you got a problem no matter how good a "Ratio" it is was the point. But the smaller the area almost the more the acoustic treatments somebody might end up needing in the end to correct it.

Can't argue with that! Too small IS too small and that size brings some awful problems of its own. I had my Maggies in a large closet once....about 11 x11x 8 and it was the worst. I had to swap them L/R to get the image correct...
10x18 is pretty small, but not a deal breaker. You're not going to get big speakers in there and you may end up doing a short wall setup and listening near-field, but for a personal listening space it can work. Don't put your back up against a wall, either.
The good news is that you have only so much wall to treat. That will keep expenses down.
The only thing I'd wish for is when starting with a clean sheet of paper, to get the fundamental dimensions right which may help minimize the expense of later treatments.