They've made two SC series receivers every year for the last 4 years. First, the 05 and 07, then the 25 and 27, followed by the 35 and 37 and this year it's the 55 and 57. I had a 35 last year for a short while but had the chance to make a quick buck on it so I sold it. Bought the 27 last month and really like it. There are rumours the X7 models have better Digital to Analog Converters (DAC's) and some other internal parts than the X5 models but I've never really investigated those claims. So, the 27 is two years older but a higher model than the 55. I've also never compared the features of the SC models released over the years because I really don't use the latest features found on these newer models like streaming, iPod and Bluetooth, Internet Radio, etc. maybe someday, but not today.
SVS, B & W, HSU, Paradigm. There are many high quality speakers out there but as Robert said, try and stick to the same brand, at least for the front three. .