Why usually a stereo amplifier costs more than a stereo receiver with much more options and similar spec?

Right now, Amazon has the RS500 for about $200 less than the AS501. Given the similar model designations, you might think the "RS" designator is the receiver version of this integrated. However, dig into the specs, and you'll see it's not the case. This unit has higher RMS power (85 W vs 75W), lower distortion (0.019 vs 0.04), and specs like peak power, SNR, and freq response are not provided for the receiver (presumably because they're not worth noting). You also need to look at how power is calculated. Manufacturers are notorious for cheating AV receiver specs by reporting power over very narrow bandwidths (sometimes just a single freq!) whereas the power for this integrated unit is over the entire usable bandwidth. So, in short, the specs are not identical.

Do these amps have a quality headphone amplifier that would be able to properly drive headphones with a somewhat high impedance? Ex. Sennheiser HD 650

The headphone jack is connected through a voltage divider to the output of the power amp and its impedance is about 450 ohms. This guarantees that HD650 will have a bump in the vicinity of 100 Hz, the headphones’ resonance frequency, which will be hard to correct with the amplifier’s bass control. For headphones, I have a separate DAC/Amp pair (Topping D30 and JDS Atom).

Does the Yamaha A-S501 have a pre-amp out? If not, can one of its two recorder-outputs be used in lieu of a pre-amp out?

The above answers are somewhat confusing, here's a straight forward answer : IF the external amp is a 'power amplifier', the answer is NO, you cannot use the rec out output as a preamplifier. Power amps always output at the rated power, say 300 watts rms. You will blow yr speakers. Examples of power amps would be the Crown XLS and the Benchmark AHB2. They have no controls, and you need a true preamp to use them. Not this unit. Now, if the external amp is a 'variable amplifier', the answer is YES, you can use the rec out as a makeshift preamplifier. Variable amps always have a volume knob, or control, which, of course, controls the power of the amp. A good example of this would be the Topping LA90, one of the best made. Keep in mind that if you decide to use the rec out output, you will bypass the tone controls and volume of the integrated amp. And yes, the phono input and selector switch will work just fine, no worries. Finally, the AS501 is a splendid unit, why use an external amp at all? If you need more power, simply buy the 701 or 801 and sell that external amp! Anyway, hope this helps.

ASIN	B00MXUCMWO
Batteries	2 AAA batteries required.
Best Sellers Rank	#17,343 in Electronics ( See Top 100 in Electronics ) #80 in Audio Component Amplifiers
Date First Available	September 9, 2014
Is Discontinued By Manufacturer	No
Item Weight	22.7 pounds
Item model number	A-S501SL
Manufacturer	Yamaha Electronics
Product Dimensions	17.13 x 15.25 x 6 inches

A-S501SL Natural Sound Integrated Stereo Amplifier (Silver)

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