The SM 450's are a relatively small speaker so they aren't going to " rock the house " but you shouldn't have a problem hearing them, unless you are sitting 15 feet from them.
You could try hooking up a CD player directly to the Onkyo without going through the computer or xBox. The Onkyo should be able to deliver the power the SM 450's need especially in 2-channel mode. You'd want to set the receiver to a 2-channel mode instead of a surround mode anyway.
I have the newer SM55 ( same configuration as the SM450 ) and use them for 2-channel music when I don't want to fire up the big speakers and the amps. I don't have a problem listening to music in my dining room from where the SM55's are, but my receiver gives them roughly double the power of the 616 in 2-channel mode.
Denon AVR X7200WA, Emotiva XPR-5, Emotiva XPA-7, Sony XBR 75" 850C, Oppo BDP-103D, Roku 4.
DefTech BP7000SC (L/R), CLR3000 (C), SM55 (FH), UIW BPZ/A (SB), UIW 75 (TM).
JBL L7 (Sur), EoSone RSR 350 (RH).
PC 2.1 setup. Emotiva Stealth 6, SVS SB2000 sub, Emotiva DC-1 DAC.