The TX-SR875 is rated 7 channels @ 140 watts/8 ohms, not 200 watts RMS. In fact, the [email protected] is an FTC rating, meaning the actual output into 7 channels driven simlultaneously could, in fact, be significantly less than 140 watts. My Pioneer VSX-94TXH is rated [email protected] ohms, and tested output is closer to 65 watts x 7 with all channels driven. Take the wattage ratings with a grain of salt.
I echo the recommendations of the others who say if you're going with Definitive for front and center channels, that I'd stick with Definity for surround and back channels. It will be easier to ensure a good tonal (timbre) match, and even though your Onkyo has individual channel equalization and automatic set-up, having like branded speakers all the way around sometimes eliminates tonal variations that EQ might not take out due to the center frequencies and slopes of the EQ.
If you already own the Bose cubes, there's no harm in using them for surrounds. They're small, mount easily, and will sound fine for surround duty. No problem with the power handling, even the loudest movie soundtracks are usually pretty gentle on the surround speakers.
YES, I consider a subwoofer "mandatory" equipment for a 7.1 or 5.1 configuration. No matter how good your front left and right speakers are, they're not going to be able to effectively reproduce the LFE channel data in a large number of movies. Consider a high powered subfoofer has INCHES of cone excursion and many hundreds (or thousands) of watts of dynamic power to handle that 15-80 Hz content, and you can quickly see that your 140 watts per channel won't measure up.
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