Close mic = lots of cab, minimal room sound; distant mic = Some cab, some room sound. The ratio of cab to room sound (and the nature of the sounds themselves) that the mic picks up will be dependant on your tone, the cab itself, the volume you play at, where you put the cab in the room, the size and geometry of the room, what the room's been constructed of, furnishings in the room, where the furnishings have been placed, the mic you use, and where you place the mic. Probably not the simple answer you were looking for, but these are all things to consider. You're probably not about to go re-arranging the room or moving to another room altogether, so the most practical things to focus on would be mic selection, mic placement AND orientation, and the placement of the cab itself.
The bottom line: An untreated room will have an affect on the guitar tone, and so will a treated room. It will be harder to achieve a good sound in an untreated (or treated, but unsuitable) room, but not impossible. Sure, you may be unlucky and have a room in which you'll never be able to achieve a good sound, but it's probably most likely that you have a room in which you'll be able to achieve something close to an acceptable sound. It really is a matter of thinking about how you go about it, and trying things.
I could offer many ways to go about doing it, but I guess it's probably more appropriate if I ask what are you actually trying to achieve? Having it clear in your mind what you are trying to achieve will probably help you help yourself as well.
Anyway... one of the first things you could try doing is unplugging the guitar and holding a finger on the tip of your guitar lead. The buzz this creates going through the amp contains a similar spread of frequencies to what a guitar does... so while doing this, chuck on some headphones, gain the mic, have the amp low enough that you can hear the sound in your headphones more than the amp, and toy around with mic positions while listening. The thing to listen for here is not just the room/cab relationship, but also the balance of harmonics in the sound the mic is picking up and trying to translate that to the guitar sound that will be coming through later. Once you've done it once, do it again with a radically different cab position. Say you had the cab pointing at a flat bare wall, point it into, say, a corner that has a couch in it.
Don't forget to play around with the mic angle as well. Find a spot where the cab sounds good, and if the room's still crashing in try twisting the mic so the angle (but not the position) of the diaphragm changes. Even just having carpet on the floor could make a big difference in your sound without having to move anything but the angle of the mic. Sure, it will sound more dull, but the great thing is that as the mic is still in the same piece of air so you'll still picking up the same harmonics. Boosting the treble a touch with some judicious eq will restore the brilliance you loose quite easily while also adding a sense of 'air' to the sound.
Work out what you want to achieve, then try different things until you achieve it. Learn from what doesn't work, learn from what does work. You will eventually figure something out.
