I agree with Valinar, speakers are most important for playing loud, as they determine this to a much greater degree than the amp. I would consider something like Klipsch outdoor speakers with the NAD. Some of these speakers have a sensitivity of 95db, and using the 40W NAD should work well. You would need 160W with the PSB's or Mini Monitor's (89db) to get the same loudness, and even more on Atom's (87db). Not to mention the outdoor speakers will have more ability to withstand the garage environment. Maybe the amp could be located inside, and wired to the garage. Actually think it should sound good, giving most of the credit to the NAD, and yes the other speakers would sound better, but at lower volume. Just think about the conditions for listening.