This really is a sad state of affairs, where have all the players with 7.1 analogue output gone?! I too have a trusty old Sony S550, just hope it continues to run for a while yet. Replacing my AV processor just isn't an option, I have a Primare SP31.7 surround processor, their current model with HDMI support is the SP32, which comes in at £3000! Yikes!
There is a few of these boxes that attempt to extract audio from a HDMI stream, which seems fine on the face of it, but you need to speaker setup, level, delay, bass management processing, which is found on these now rare bluray players. Very few AV receivers/processor will digitize the 5.1/7.1 analogue input and perform any DSP on it, most purely pass it straight though and act as nothing more than a volume control.
Looks like the Cambridge Audio and Oppo players are the best options, albeit not very cheap options. Still, their universal disc support is certainly very tempting (SACD, DVD-Audio, HDCD etc) and obviously much better DAC's than our basic Sony S550.