Whoohoo Lego !
Seriously though, a question. Are you referring to the 60s designed for in-wall placement or are you placing actual, freestanding M60s into a wall cabinet? If so, you may suffer a bit of negative effect as their design, including rear ports that need some room to breath, were not truly intended for a cabinet placement.
Honestly though, I'm not qualified to tell you how much effect or how much it will matter to you. I'm just repeating some of the stuff I have learned myself from this forum and some other sources.
