Re: can someone explain this about amp wattge?
OK. So your receiver is a healthy 75 WPC. I do believe that it should be sufficient for you to get to reasonable listening levels. Your surrounds are spec'd at 2 dB lower than your mains, and 1 dB lower than your center. Also, your mains are rated at 4-8 ohms, from what you have posted, and your surrounds at 8 ohms. I understand that the impedance of a speaker will fluctuate over the frequency spectrum, but speakers are usually spec'd at a certain impedance. It's also interesting that the speaker manufacturer has chosen to spec some of its speakers over an impedance range (e.g., 4 to 8 ohms) and some at a specific impedance (e.g., 8 ohms). Anyway, the sensitivities of the speakers do not constitute a large disparity, in my opinion. I'm not sure what to think of the difference in impdeance specifications.
It sounds like you are having trouble calibrating the five speakers -- you can't get them to perform at the same relative level during set up tests. I think that you should be able to get them pretty close. Let me ask -- how are you calibrating them? What tone/noise source are you using? What SPL meter are you using? In what mode are you using the SPL meter (e.g., "A" weighted, "C" weighted, "fast" or "slow" mode, etc.?). Are your surround speakers very far from the testing/listening position when doing measurements? What "mode" is the receiver in when doing the measurements (e.g., "stereo", "Dolby Digital", some other processing mode, etc.).
When I do this, I generally start with my "trim" level for my mains at 0 dB and turn up the volume until I read 75 dB on my SPL meter. Note that the 75 dB reading is somewhat arbitrary; it's a commonly used reference number, but we could also use 80 or 85 dB as well, as long as all the electronics and transducers involved can handle it (which I believe they should in your particular case). Once I read about 75 dB on each of my mains, I go to the next channel (usually the center) and adjust its trim until it reads 75 dB as well. Repeat for the surrounds. Currently, since I'm bi-amping my mains, I have to turn the other speakers quite a bit hotter. I think my center is at +6 and surrounds at +4 dB. I have a swing of +/- 10 dB in each channel's trim level, so I'm not really getting toward the limits of my preamp.
If you are doing that, and you are turning the surrounds trims to their maximum, but you can still only read 72 dB when the mains read 75 dB, then you can drop your mains to, say -5 dB, and then you should be able to achieve a common reading of 75 dB for all channels in the system. From there, you should be able to turn up the system as a whole beyond the 75 dB level, but in reality that might be too loud for casual movie listening. I rarely listen to movies at "reference level" where I have done my setup (reference level being 75 dB in my system -- same as yours).
I don't think the problem lies with your receiver running out of power. Previously, I used a Denon receiver that I bought in 2000, rated at 70 or 80 WPC. I didn't read up on your particular receiver, but Rotel is a well-respected brand that I suspect would be "conservative" on their power ratings. In short, I believe your Rotel is superior to my Denon in terms of power, regardless of what ratings they have. I'd also imagine that the Rotel will probably be spec'd with "all channels driven." Do you know if this is the case or not? Either way, when you are doing a setup, all channels are NOT being driven, so you should have sufficient power to each channel during the test. So, getting a bigger amp isn't necessarily going to fix that problem. Also, it's my opinion that the surround channels aren't necessarily taxed all that much during "normal" movie sequences; therefore, you're really never going to run all channels at their max. Even during massive explosion scenes, not all channels are running at max. Furthermore, if you're at all using a sub and crossing your speakers over, a lot of the big power that's needed for bass will be diverted from your receiver and to the sub's amp, again taking power requirements off your amp.
I think something's wrong with your setup or perhaps connections. If you want to resolve these problems by buying a bigger amp, you might be disappointed. If you're just itching to buy a new amp, and a new toy, then hey, I totally understand that -- more power to ya! But I believe that you have a very capable amplifier section in your Rotel (especially, especially since you indicate that you don't want to get super loud), and that once you resolve these issues, you will be happy. If still want more power after that, go for it!
Let me know what you think...