to be honest with you all a grado driver needs is an Impedance converter on a tube stage or a solid state High current Buffer on anything rather than a "true" headphone amp even though that is what the choices mostly are.
The Grado headphone do not require any actual
voltage gain unless you are trying to drive the cans from a portable CD player or an iPod type player which in reality can drive this class of headphones to a more than acceptble volume level but to get a proper music experience you
do need proper impedance matching,good current drive into a low impedance load and the sonics must be smooth across the board rather than "etched" sounding so anything reviewed as being "smooth" in sound is a candidate (funny how what was once known as good sounding and accurate is now called "romantic" or "lush" while what was formerly known as "too bright" is now called "detailed" ...........).
that satisfies the sonics but still means you are "over amped" in the gain department with just about any amp that does not allow for a low gain adjustment and also usually means unfortunately a very limited volume control range on the amp,piss poor left/right channel tracking on the volume control and an increased overall amp noise level.
The limited volume range is all about trying to cast off the extra gain and because of this means a large amount of signal attenuation with most amps so bottom of the volume control rotation where tracking sucks the most along with a hair trigger response when being adjusted due to the limited range of useable rotation.I like a "usable" 3:00 to 2:00 range which enough range above and below for very low or extremely "hot" signals
Bottom line the requirements for a Grado specific headphone amp would be :
1-Pure class-A operation.Being sensitive to even the smallest of signals the low level parts are far more audible than with most headphones so moving bias audible
2-Low Gain or ZERO gain.A happy medium case would be X2 (+6dB) to retain the ability to listen to low level sources while not losing volume control range at the top for high output devices.This would use a dual pot in the range where tracking is at its best and also alow for finer adjustments rather than a "twitchy" volume control where small adjustments have huge results
3-A very low output impedance or you lose bass drive even though the amp may "sound" like it has more bass detail because it is being leaned out.
4-Eceedingly smooth in the upper octaves.Bright electronics and a Grado mid to upper end will send most humans running from the room swearing of the Grado experience for ever !
this goes to internal passive parts selectio along with Class-A operation
Tube Amp "Pro" :Wider sound stage,more "air" around the instruments,a much more life like representation with natural instruments,wash with electronic instruments
Tube amp "Con"Can have higher noise which will be heard as background hiss during the blank spots between music cuts though rarely audible with music
Has a higher output impedance and limited current drive so must have either a single stage or actual power tube drive or if a small signal triode either configured as a cathode follower or MU stage or White follower or maybe even a hybrid SS/Triode follower stage or it will not properly drive the low impedance.
will be expensive for the good ones usually
Solid State Amp "Pro"Way lower self noise so will be dead silent between songs/notes unless a really piss poor design
Higher (usually) current drive into low impedance loads.this translates into very tight and very impactful bass.not important as such for real instruments but with electronica or rock will get the Grados moving 'air" at the bottom like no tube gear can that i know of
Cheaper by far and will usually have a smaller footprint on the desktop/shelf
Solid State amp "Con"
A much smaller sound stage and less "air" around instruments which makes for an overall smaller image,one that with acoustic instruments seems to suck the life out and take a performance from "I am there" to" this is a recording".Solid state is mostly two dimensional where tube gear is more three dimensional
What you gain in bass impact you lose in emotional content from the middle up.Rather than having "flesh and blood" attached to the music you get the plain brown wrapper version of
Tough choices and why a hybrid
tube voltage gain stage with a
solid state current stage output may be the ideal within reasonable cost lmits and why maybe something like the X-Series in a lower gain version would have merit though never having actually
heard these amps just conjecture.
In my personal case i have several amps that all work up to my personal expectations but are as dissimiliar as amps can get because of where in my "systems" the amp resides.
I have a dedicated traditional "headphone amp" that is a two stage affair with a multi-tap output transformer.This transformer has a higher step down rate as the primary-to-secondary impeance ration is increased so in effect reduces the output levels as the impedance is lower making the amp universal (high step down+low gain for Grados,smaller step down +higher gain for other cans).This is my "main" headphone amplifier per se.
I have a simple "resistor and switch in a box" interface on my primary power amp,a single ended 300B amp that puts out around 8 watts
I use the ASL UHC matching transformer-spker/hdphone switch on the output of my high power monitor amp,a PP 6L6 "pentode/triode" amp.gets around 20 watts pentode I guess
I have a Szekeres "voltage follower" and several monolithic "buffer in a box" voltage followers at varios points in my record/playback.The Monolithic "buffer in a box" serves double duty as my long line driver so has a line/headphone selector for choosing output.Zero gain,just current baby
Which one is best ? If i could pin that down i would have a singe headphone jack in my system but instead find each has a strength depending on what I am trying to do and where "in system' the amp resides.I can say each of the above sound more alike than different since the actual drive is the same
1-low gain two stage voltage gain/output drive tube headphone amp
2-High current actual power amplifier with attenuation/impedance ematching network
3-High current output zero voltage gain interface
Grados are the only headphones I know intimately that can be driven to loud levels by anything you can put a headphone jack on so any increases gained by an amplifier are in the "quality of sonics" department and not volume level.
The problem is an amp designed from the start to be Grado Specific will usually be very lacking in the volume department when driving any headphones in the "above 60 ohms" category so universal operation is out unless you have either selectable gain or a multi-tap output transformer on the output.
I know this does not actually answer the "what amp' question but I am not one to recommend specifics unless I have actually heard the combination over time.most modern amps sound good but it comes down to system and taste on what is actually "best" so I gave a "these are the requirements and his is why" type answer.hopefully it helps
Rickster