I am looking to buy the CM9 theater from Bowers and Wilkins and run it with the Onkyo 709, because it has a phono input. I will be running it in a room that measures 10x20.
Would it be a big change if I got a seperate amp? Like the Emotiva XPA 3?
A dedicated pre-pro and amp combination is better than a receiver because they have only one job respectively, process the signal and amplify the signal. Since the manufacturer does not have to make a MSRP based trade off between pre-pro section and amp section and cost of implementing "features" like S-Video connections, Satellite Radio, etc., the dedicated pre-pros and amps can potentially have much better implementation of their respective functions. Further, the consumer can choose the exact combination suitable for the need. IMHO, a well made $1000 7ch amp will achieve better amplification than a feature packed $2000 receiver and adding a $1000 pro-pro will give more important/usable features than the same receiver. That said, one looses the convenience of “one box does all” and things can get expensive since you are now in audioholic freak territory
. The question being, is it worth spending the money on a dedicated pre-pro + amp setup when a receiver does it all. Well, the pre-pro can be upgraded over time and a good amp can become a heirloom
.
3ch amp like the XPA-3 is a good compromise between the dedicated vs receiver benefits. The front and center channels get a majority of the signal and therefore these speakers consume most of the receiver's power. By contrast, the side and rear surround channels and speakers perform considerably lighter duty (some multichannel music and using floorstanding speakers for surrounds being an exception). Since you are using the smaller surround speakers, by adding the dedicated 3ch amp for the front and center speakers, any mid range receiver will easily drive the 4 surround speakers. You can even get a slightly less powerful receiver, say 100 Watts/ch instead of 125 or 140 W/ch.
And, there is your very pertinent question, will you really make out any difference? For the answer, we need to look into the speakers being driven. For very efficient (high sensitivity like Klipsch RF-52 rated at 96dB/W) or easy to drive (impedance vs frequency plot not dipping too low like, Martin Logan Summit, 4 ohms nominal, minimum 0.7 Ohms @ 20kHz) speakers), there is less discernable impact by adding power and headroom to the amp. The CM9’s are rated at 89dB/W. So they are on the lower middle ground of sensitivity. Meaning, when compared to the Klipsch, more continuous power will be needed to get the same loudness. And even more revealing revealing is the bold part below, from a CM9 Series Review article’s
measurements of CM9,
The CM9’s listening-window response (a five-point average of axial and +/–15-degree horizontal and vertical responses) measures +0.83/–5.12 decibels from 200 hertz to 10 kilohertz. The –3-dB point is at 60 Hz, and the –6-dB point is at 49 Hz. Impedance reaches a minimum of 3.41 ohms at 116 Hz and a phase angle of –68.80 degrees at 66 Hz.
It means, when playing a signal of 116Hz (+/- is tough to say without a full plot), the amp will be put through its paces. If you are playing media with a lot of bass (content above sub crossover setting of 60-80Hz or running when the CM9 as "Large"/full range) the amp's capabilities will be of significance.
So, do your speakers need a big beefy amp. If you intend to run the speakers as “Large” meaning send them full range signal, the answer is, a resounding YES!!! If you are going to use a sub and never run the CM9’s as “Large”, you might be able to get away with the Onkyo 709. In the latter case, you will just have to wing it and decide for yourself.
Sidebar on headroom: Assuming the amp does not have issues like coloration, unacceptable harmonic distortion or inter modal distortion, etc. (any good amp will not) The greatest impact of quality amplification is in ability to cleanly dump a huge amount of power when called on. This is called the "headroom" and to oversimplify, difference between continuous power and peak power rating. Headroom is a fancy way of saying, if there is an explosion in the soundtrack, can the amplifier send the speaker a sufficiently amplified signal without distortion, to recreate the explosion sound at the desired loudness,
without distortion. Running out of headroom results in clipping which can damage the speakers and/or the amp itself (almost all receivers and amps come with preventative measures, such as, soft clipping capabilities or receivers that shut down the amps. When the receiver shut down, it is the poor thing telling the listener that it can longer amplify the signal to the level needed by the speakers and so it is shutting down before something on the circuit boards gets fried). Among other things, having a big bank of quality capacitors gives good headroom and a dedicated amp will certainly have them and to be fair, as will a well made receiver, usually the higher end ones.
