Perhaps the delay in getting a response is partly due to the season
Anyway, here's what might be a solution to the floating shelf problem...
Concealed Shelf Support, Architectural Ironmongery
Can't comment on speaker choices as I dont have the experience, I do know that richersounds sell some nice looking Denon mini systems sans speakers for about £150, with DAB, and though the depth is listed as 328mm and the shelf brackets I linked to state 300mm max, I would think an extra 40mm or so wouldn't break them, given that they support 25Kg's.