Amps and Receivers

What is the difference?
 Strictly speaking the amplifier's only function is to accept a source signal from another device and amplify that signal for output through one or more speakers. 
Whilst other functions such as selecting the input device and controlling the volume are controlled by a Pre-Amplifier.
 
The separate pre-amplifier and amplifier remains much beloved by audiophiles but is rarely employed in home theatre due to the additional expense. Almost all amplifiers intended for home theatre combine both these functions in a single unit.
 
A receiver also provides pre-amp and amplifier functions bust also incorporates a tuner for receiving radio broadcasts. Receivers are now more common than straight amplifiers reflecting a trend to reduce the number of devices, reduce overall power consumption and simplify remote control operation.

Many home theatre amplifiers and receivers incorporate video switching and digital signal processing capabilities. Capabilities vary widely according to price and manufacturer.
 
Weight Tells ... 
The most critical component within an amplifier is the power supply. A heavy amplifier means the transformer is built from high quality materials.

 
Examine the outer dials and switching devices
Buttons Dials and Switches should all have a pleasant and precise motion. Poor quality external controls are a sure sign that poor quality components have been throughout the construction of the unit.
 
Signal to Noise Ratio (SN)
 This is the ratio of true sound to background noise.  Once you have the Receiver hooked up to the Speakers of your choice turn up the volume with nothing playing on the system.  You shouldn't really hear anything.  If you hear any background "hiss", you're not getting true sound.  The sound is being lost due to the poor construction of the unit. 
This rating is measured in decibels (db), the higher the rating, the truer the sound

Total Harmonic Distortion (THD)
Measures the amount of sound distortion at higher volume levels. Sound should remain clear even at high listening volumes
Rated as a percentage, Lower numbers indicate less distortion

Efficiency Rating
Efficiency is the ability of the unit to transform power into clean sound.  If you get a 200 watts per channel Amp with an efficiency rating of 50%, you're really getting a unit that produces 100 watts per channel, background hiss and a lot of heat.  Manufacturers will sometimes boost the watts but forget the efficiency knowing full well the average consumer goes for the unit with the most power.  This rating is given in a percentage.  The higher this percentage, the better.
 
Power Rating - Wattage Root Mean Square (RMS)
Measures the capacity of the amplifier for sustained power output.
Rated in Watts, be certain that the RMS, not peak value is quoted
 
Dynamic Headroom
is the ability of the Amplifier to handle sudden increases in sound levels.  This is most important in a Home Theatre application as movie sound tracks are always bouncing from quiet to loud at any given time.  This rating is given in decibels.
 
Do I really need Dolby Digital
This depends on what source material you'll be playing on your Home Theatre system.  If you're going with a VCR as your main player for movies, an Amp with Quadrophonic sound or Dolby Pro-Logic will be sufficient.  If a DVD player is what you're using to play your movies, you'll want Dolby Digital 5.1 (see the SOUND FORMATS section for more on the different types of sound). 
You can also get a Receiver with a DTS processor using the 5.1 channel output to re-create this effect.  If you wish to go this route, I believe you're better off getting a Receiver or Amplifier that is DTS ready (not on board) as the after market DTS processors far outweigh the built in type.  Remember, your DVD player must be able to play DTS sound if you want it all to work.  My opinion:  DTS is a valid format but, consumer wise, is not well recognized.  It is something to consider for a future upgrade, which is why I suggest getting a DVD player that can play DTS and a Receiver or Amp that is DTS ready.  This way, the choice is yours.