I can help best if I see a pic of each room.
A large room presents different issues from a smaller room. To start with, in evaluating the suitability of a room for use as a music room, you're balancing the ratio of the direct sound coming from the amp in this case, to the sound bouncing around the walls, floor and ceilings of the room.
The direct sound is what's coming from your amp and speakers. The reflected sound is that direct sound, bouncing around the room in very complex ways.
A large room has longer decay times, plus the level of direct amp sound you're hearing, as compared to these room reflections, is decreased.
Thus, in essence, what you're hearing in addition to whatever room modes there are, is a whole lotta reverb in addition to the guitar amp, but the larger the room, the more the ratio of reverb to direct sound increases. It's as though you recorded a guitar amp, and put too much reverb on the track. It becomes muddy, diffuse, and boomy in the low end.
Think of it this way; a gymnasium sounds different from a concert hall sounds different from a small recording booth.
You're hearing a lot more of the room's reflections off hard parallel surfaces in a gym; a concert hall is large, too, but it has specially designed surfaces, angles, seating surfaces, and other treatments designed to make it sound "good;" you have a lot less room sound in a recording booth, even though it, too is specifically designed to sound good, it's different from the concert hall because it's designed so that the sound of the instrument itself is very direct.
A living room is going to have longer reflection times, and significantly more reverberation compared to the direct sound of the amp, than a small bedroom. The surfaces and furnishings, including carpeting, may matter quite a bit. These longer decay times can make an amp sound very diffuse in the room, and the bass can become quite boomy as it bounces off various surfaces.
Placement: An amp against a floor doubles the apparent bass due to half space reinforcement; put it against a wall on the floor, and you double the apparent bass with quarter space reinforcement; in a corner, and you've added eighth space reinforcement of the bass. In a larger room, longer wavelengths of low frequencies can develop more than in a smaller room, too. And bass reflects off walls and ceilings just like every other part of the frequency spectrum.
Putting amps on a coffee table can help, or it can make things much worse, depending on the table's construction, the speaker cabinet, and its placement in the room. With some furniture, there are times you're resonating the table itself and creating a drumhead type of effect.
You're not going to duplicate the sound of a small room in a large one without a lot of careful work to control reflective surfaces. But there are things you can do that will help.
The question of whether your spouse will let you do these things is of course the biggest hurdle...
In my old house, I had a grand piano in a living room with hard surfaces, a two-story ceiling, two story windows, and a large marble fireplace surround that was about 12 feet wide and two stories high. It was impossible to record the piano in that room, there was simply way, way too much reflection. Treating the room properly would have completely pissed off my wife. So it was not going to happen. That piano never, ever sounded right in the room. Neither did a high end hi fi system. I moved the hi fi to the den, where it sounded great, but the piano was too large for the den, so it lived where it wasn't the best sounding room.
This is what life is about. Things in rooms they don't belong in.
I think my wife would sometimes like to move me out of the rooms she thinks I don't belong in.
