Are you looking to replace the Sony??? Presumably not as it was a fine player.
Better? I have no idea. What purpose is it for? Multichannel (which not all of these are)? Whatever...here's my findings to date! Sleuth-Spot's on the case!
The Oppo could be seen as the easy option; great budget deck, well reviewed all over the place and well supported by Oppo.
The Sony 900V gets a great review from TNT Audio, fast and detailed over warm and smooth apparently with a leaner bass. I like what I read from that site and they're enerally pretty on the money from what I've seen. The 900 originally went for around 700euros, so at the time, that may have been around £400 or so.
The 9000ES dates from circa 2001, built like a tank and allows you to select audio only (by switching off the video circuitry, so a "pure direct" kind of feature) as well as featuring apparently audiophile grade components throughout. Sony hadn't long come up with SACD, so they were pushing it hard still here. The reviewer on the site I found (US based, not Stereophile though), felt audio performance in general was well above mainstream for such a unit, reference even (for a DVD player anyway) and SACD was very decent. Another reviewer (John Sunier) goes further when he gets all the waffley preamble about his system and running the player in for 400 hours (please, get a grip John...) and rates it as exceptional apparently. Better indeed than the Sony 777ES according to him, which was a step up from your 555ES. Leaves the CD replay behind supposedly, so maybe worth trying this one out?
Here's some stats on the 9000ES player:-
"24-bit/96KHz Digital Output.
24-bit/96KHz Current Pulse D/A Converter.
VC 24 Plus Digital Filter.
Two Audio Master Clocks.
Audio Direct Output.
Separate Audio Circuit Board.
Twin R-Core Power Transformers.
Multiple Power Supply Secondaries.
Audiophile Parts.
Super Audio Compact Disc
Plays Back Super Audio CD (SACD) Software.
Direct Stream Digital 1-Bit Coding.
Sample Rate of 2,822,400 Hz.
Accurate Complementary Pulse Density Modulation (ACP) Stage.
Separate CD and SACD Low-Pass Filters.
Construction
Precision Drive FX Mechanism with Dynamic Focus Pickup and Tilt Mechanism.
Copper-Plated Frame and Beam Chassis.
Off Center Isolator Feet.
Hermetic Shutter.
BMC Mechanical Deck.
Mechanical Deck Isolators .
Aluminum Front Panel.
High-Speed, High-Precision Servo DSP."
The Yamaha is a pretty recent effort, so assume that current data or reviews are easy to be found in mag listings or Yamaha's site.
Think my money would be heading towards the higher end Sony, but expect to pay around £200-£350 at a dealer's. Loud and Clear had one (in Edinburgh) last year; £350 asking price used, but mint.