Oof. In acoustics (as well as a lot of other areas) there's this thing called the inverse square law. Basically, SPL decreases inversely proportional to the square of the distance from the source. So, go from 1M to 2M and the SPL drops -6dB. In open space. In a real room it's more complicated. The direct sound drops according to the inverse square, but the reverberant field does not. Also, low frequencies use the room surfaces and load up like horns: freespace = 1 , hemisphere = 2X (same acoustic power into half the space), quarter space (intersection of 2 walls) 4X; eighth space (2 walls and the floor) 8X. And it's all frequency dependent. This is why speaker placement matters so much.
But SPL at the listener's ears is a combination of direct, reflected, and reverberant levels. Here, halving the room volume, all other things being held constant, the same energy input into the room will be doubled per cubic foot, and that's 3dB.
So, yeah, the dealer is kinda correct, but in an oversimplified way. If your room is acoustically very absorbent it might take even more power. If you load your speakers closer to room boundaries, we already covered that. At best, the answer is a qualified maybe. In any event, listening a realistic levels in a good sized room is not something most HiFi speakers will do, they just can't move the air. You need some big Klipsch or real JBL monitors - S4700 or such. Full disclosure, I've never been a Klipsch fan, and JBL L-100s are simply terrible. But the new JBLs 3900, 4700, are simply spectacular.