Yamaha AS500 or Onkyo A9155

The 100 watt rating of the speakers is the continuous power at which the voice coils will melt and go to heaven. You don't need 100 watts of amplification, any more than you need a 130 MPH car for a set of tires rated for 130 MPH.

The Diamond 10.1 is excellent and I recommend it; the 10.2 has better bass extension and is also very nice for slightly more money.

The Diamond 10.1 does have a low sensitivity of 86 db per watt, however, which is on the low side; that is the real issue. This means more power is required for peak sound levels. 65 watts at 4 ohms is what the 9155 is actually rated at, and that is is probably enough, but I would suggest slightly more (the 10.1 IS a 4 ohm speaker).

For a better-sounding amplifier which also has enough power, I suggest that you consider the Music Hall A15.2. This is an outstanding amplifier which sells for $499 in the USA. I don't know what it sells for in the UK.

If you decide to go with Onkyo, the A9377 might be your best bet, with 90 watts per channel.
Edited by commsysman - 12/1/12 at 10:23am

The compares with almost every other amp or AVR with what appears to be a higher power rating which puts out 90 or 100 wpc.

100 wpc is only 3 dB more power than 50 wpc which in a casual comparison is just about a noticeable difference. Its audible, but not dramatic. It would take a 500 wpc integrated amp (most of which are made of unobtainium) to sound twice as loud as 50 wpc.

Speaker power ratings are like the ratings on a circuit breaker in your house. If you have a 15 amp circuit breaker and an appliance that only draws 7 amps no harm done. If have a speaker rated at 100 watts max, and only a 50 watt power amp, no harm done.

You have several approaches that can yield greater loudness with the Onkyo amp that you are looking at. Or, you can take my advice and get a 5.1 AVR instead, probably for less money and still at least as good sound. While every AVR I look at seems to have a bell or whistle that seems useless, they generally have some useful features like digital inputs, bass management, and outputs for subwoofers and center channel speakers that can facilitate useful upgrades to a stereo music system.

(1) Speakers with higher efficiency - for example the Infinity Primus 163 is also highly reviewed by SP (predecessor P162 - about the same), on sale at Amazon for a very attractive price, and has 90 dB/W sensitivity which is only 3-4 dB more output, but it is like doubling the power output of your amp. I have a couple of pair of these and they sound really good, particularly with a good subwoofer.

(2) Use an Amp with true bass management (AKA an AVR) and add a good sub. The sub will lead to a dramatic decrease in the amount of amp and main speaker power that it takes to get satisfying bass and good dynamics.