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

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.

The ceiling being 2x length is a problem.
Enter 'room mode calculator' in google.
Rooms have multiple resonant modes. Generally, they are
2-wall.....3-wall and 4-wall. Each is less important than the one before it.
The 'ideal' (from a math standpoint) is PHI... 1.618:1
So a room 10'x16'x 26' is pretty good....but that is just math, not taking into account other factors, like furniture, construction, speaker placement and type, doors....etc/ etc.
There are some big fans of this number but most won't tell you how they derive there 'formula'.

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.

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.

Any basement room will have concrete or stone under whatever is used inside the walls. Floor is likewise concrete, no matter what you are walking on.
Also, you never mentioned the ceiling. Open joists? Drop ceiling w/acoustic tiles?
With 9feet of ceiling you might be able either build a subfloor or treat the ceiling....or both.
It's going to be a pretty LIVE space unless properly treated.