what makes some amp better than others?

Defining your use of "better" would help, but working with the general use of the word here is what I find in a good amp.

1.)  Low harmonic distortion ( THD < .002 % )

This gives a nice clean signal without distorting the input waveform

2.)  High signal to noise ratio ( SNR > 98dB )

This allows the noise floor to be low enough to prevent distortion and other adverse effects )

3.)  Larger frequency response ( FR: 10Hz - 30kHz )

At minimum you want to include the entire human audible spectrum which is typically 20Hz - 20kHz, but I find the wider the FR is the better.

4.)  Lower output impedance ( X < 5 Ohms )

Anything else higher can lead to, call it atypical, frequency response issues along with distortion

5.)  Output power at 1Khz ( P > 250mW )

This is something I look at to make sure the amp can adequately drive the many types of headphones out there.  Due to headphones having not only different impedances, but sensitivities as well, the higher the output power up to 1W the better.  Any higher is just ridiculous and too much power for most practical use headphones.

Those are the 5 things I look for in the specs in determining a better amp.  The next thing I do is check the physical design including how the power supply is implemented as well as functionality.  The final test is to listen to it and compare it against what I already have.

Thanks to nick for cathing my typo.

headphones require too big a range of signal for one amp to give good S/N with sensitive iem and drive inefficient orthodynamics to reasonable levels with any dynamic headroom

many amps recommended in head-fi threads are not suitable for the headphone - just a reflection of fanboyism, FOTM

any SS amp should have adequate frequency response, can have low output impedance - which can be "tuned" by adding series impedance  - tube amps often have higher output Z to start with - can't be made lower

but good  S/N, dynamic range can only be managed with amps designed for a range of headphone sensitivity, Z

amplifier with gain switching can help, but it would still be silly to use a 25 Vpk, 1/2 Apk output amp needed for an orthodynamic with sensitive iem that produce 120 dB SPL with <500 mV drive V

mis-matching headphone, amp drive requirements, abilities, gain means either hiss or clipping, transient distortion on dynamic music peaks

mostly imagination is the best scientific explanation - how many "comparisons" are done with any attempt at level matching, blinding, perceptual anchors, controls

absent noise, clipping or gross distortion levels the major determining factor in "sound" is frequency response - decent amps shouldn't have audible differences in frequency response

some do have inadequate signal coupling caps giving poor low frequency response, added series output impedance makes an amp sensitive to the headphones' impedance vs frequency variations

when these details are equalized even Stereophile's "Golden Ear" reviewers couldn't tell a $600 SS amp modded by Bob Carver from their own 4 figure US$ "SOTA" tube amp with their own choice of speaker, music

head-fi reviews, recommendations, "consensus" views of amp "sound" are better studied as social psychology - egged on by the Audiophile marketing press in ignorance, denial, and even active hostility towards the real results of psychoacoustics, engineering

Quote:

Originally Posted by Hughkk 

that's interesting,

u think that after certain price point, a SS amp and a tube amp will sound the same?

Price has nothing to do with quality, and so long as audio remains this unscientific and exclusive it never will.

A cheap tube amp designed for transparency (and that measures well) will be transparent. A five digit solid state amp designed to match the stereotypical "tube sound" will not be transparent.