NuForce amps have a unique design where the heat generating sources are conducted by heatsink directly to the case and there is no ventillation holes. Therefore you can
1) stack two or three amps if you have an open shelf with good ventillation
You should place the hotter amp on top (the one driving woofer) and the tweeter driving amp at the bottom. If you have 5 amps, then stack them in a pyramid form instead of 3-2 columns
2) place the amp vertically on its side. This is the most compact way to pack as many amps on your shelf. May be we should provide some kind of mounting kit

. As one customer has found out recently, this is the best way for maximum heat dissipation.