optical/coax/spdif cannot carry TrueHD or DTSMA. To put it simply. Well, optical can handle 2.0 PCM, and the core stream of DTS.
If your receiver is 5.1, you can't get 7.1 ANYTHING. Think about it.
550 is not the right choice in any case. The main advantage over 350 is the mch analog support, which I dont think your receiver has. OTOH, the 550 can send out MA as unpacked to a receiver that accepts mch pcm over hdmi. The 350 cannot. Also, both Sony players require fiddling with menu to switch between 5.1/6.1/7.1 I believe. You would also need to fiddle if wanting to output between optical and hdmi.
I know that's overly technical, but just wanted to outline some issues with Sony players; things you can further be aware of.
Stick with Panasonic, especially if using optical is what I want to say.
The Sony players also cannot recode MCH pcm into lossy dd/dts. Therefore, on one of those rare Blurays that are not accompanied by a lossy track, you will only achieve 2.0.
Long story short,
1. you need a new receiver.
2. 550 is most likely unnecessary, even with new receiver. You can save some by getting a Panny 35, and not have any headaches. The 35 can decode MA and send as mch pcm if needed, and I already pointed out that the 350 cannot, and nm the fiddling headaches. 550's benefit is for mch analogs.
the only downside of the pana players is that they cannot be forced to output dvd at 480i for those who prefer to deinterlace and scale elsewhere
I have a Denon 3300 receiver. Can I get Dolby True HD & DTS HD MA through a optical connection? Can I use the 7.1 Sony 550 output with my 5.1 receiver?