Hmm...
According to your descriptions, I agree that the noise floor of your Yamaha RX-V625 seems rather high. However, it also seems that you need to do more experiments to pinpoint the noise source in various listening scenarios, before you conclude that a receiver upgrade gives you a definitive benefit. Just FYI, I connect my X-Box to the 45TX receiver through Microsoft's own digital 5.1/component video dongle, and I don't get an audible noise floor (of course, if I put my ears directly to the front tweeter, I do hear some hiss, but it is utterly silent from the listening position).
With regard to the occasional vocal sibilance, I suggest you play around with the speaker toe-in -- especially try decreasing the toe-in so that the L/R tweeter axis cross way behind you. Pay attention to both sibilance and imaging/soundstage.
