Dude, i am not sure where to even begin... i will try.
320k mp3 is ok as long as they were properly ripped and compressed with the latest LAME encoder using -b 320. This os the new "insane" that turns on every possible feature that LAME can offer without regard for HD space. I respect it because it actually lets me do certain things with my decoder and output plugins in Winamp, but more on THAT later.
YOU NEED AN AMP for these. To anyone that wants to go out ad buy a pair of these remember, they are professional headphones. They are designed to spread the freq. spectrum ion a mostly even way, to be durable (more on that too) and.... they are designed to be plugged into a console or some other amped quarter inch plug. Probably by JA(or is it JR?)5xxx opamps because they are the most widely used in pro equipment. These are the most widely used because they can put out a lot of power. I said power because I want to point out there are two factors at play here, voltage and current. A lot of the opamps in crappy sound cards can't give enough power. Spec wize you might ok on voltage or current, but ohms law is at work here and what these phones need is power. I do not have the equipment to figure out which one more, but I have a good feeling these are more current headphones than voltage. This is just a guess.
The passive attenuation is good, as you noted, and you also should note that in a pro enfironment stuff gets knocked around. Trust me, these can take it.
i have other pro phones and even the XB700s, also the v700s that were the source for the drivers in the XB700s, The v700dj phones fall apart after a year, and trust me, these Shures won't. Sony has a nice driver in the v700, it is efficient, and something the Shures aren't, so this is why I said they need an amp. The XB700s are great cans, and insanely efficient and natural, but not designed for a pro. They could be used by one as they are generally tonally accurate, but the short cord and no 1/4" screw adapter is a detractor.
You will like the XBs.
Back to the Shure 440s. In a pro environment something else is important, which is the ability to reveal what normal headphones will not. Every part of your chain is going to be cut apart by these, first you are using lossy files, 320k and possibly LAME but still lossy, and it is the job of headphones like these when monitoring from them to be able to tell the difference between a really good mp3 and a 24/96 uncompressed mix. If not, they would not be used by any pro. Your source is an integrated sound card, where power is lacking, but what is worse is unless you were using ASIO or Kernel Streaming to bypass the kmixer, if you are on XP, or just ASIO in general, you are going to get the song plus everything else that is supposed to be mixed in from the other digital sources on the card's chipset. Mute doesn't mean mute. This brings me to your attenuation and EQ. This must be passive. That is, you need to have the best signal form your card you can get, sent to a pre, or line stage, to be passively attenuated (turned down.) That is right. Full power from your card at all times and adjusted with an analog knob. Digital gain adjutment is actually BITRATE adjustment. Lowering the volume lowers the dynamic range and birate, and with 16/44.1 digital volume contol can tear up a good mix. WEven if you had thee songs in uncompressed 24/96, your internal card will probably (badly) resample it to 48khz before resampling AGAIN to whatever you ultimately chose. This is because these cheap cards have DSP chips that can only work on 48khz signals. It is actually better to use a quality software resampler on your output to get it to 48khz before it hits your card, as the chips used by the card usually have horrible artifacting and truncating. In Winamp, also Foobar, you can get plugins that will allow you to adjust the decoder and output, even using ASIO, which can dop a very nice job of resampling before sending to the soundcard, this will do wonders for any soundcard that is doing hardware resampling. Creative only stopped doing it in their consumer cards with the X-Fi, and one model of the Audigy 2. Both must be manipulated by someone that knows what they are doing to ensure bit-perfect output from the card, or to it's own built-in DAC.
A bit-perfect output with an external DAC made for quality of sound can transform many mp3s from 320k down to v0, even v2, into a nice signal to send to your headphone amp, or whatever you are using as one, when you have things setup right. Even then you must be careful because technically an mp3 is a 32bit float file, and can be decoded into a 32bit signal. This will ive a lot of headroom to use digital volume adjustment, but it takes a nice pro card, or nice external interface to accept this, plus, you must decide what kind of dither, if any, you want to use when manipulating all this 16bit stuff for resampling, if you choose to do so. The best mp3 decoders will decode at higher than 16bit and must dither down to 16 for output, so you need to know ab out rectangular, triangular, gaussian dither, as well as noise shaping, all this is related to trunbcation and would take along time to explain but some choose to use a lot some a little, and in certain cases, it may not be needed. It all depends on how your mp3 is being decoded. External s/pdif and usb DAC's can take this signal and send it once it is analogue, to your source, which is where you plug the phones into. It can be an ipod to a mixing console and all manor of in betweens such as tubed head amps and SS head amps that have the power, along with proper opamps, to truly take that signal and send it to the phones.
When done properly, the Shures sound great. They need power, and they are built to reveal, so 320k LAME through a good decoder and output to a good DAC then amplified, would sound not bad, but FLAC or .wav, really any lossless way of storing the file is truly the way to go. You want bit perfect reproduction, and it can only be done if you know what you are doing. These phones were not made for windows media player, internal sound cards, no thought to your decoding>output chain, or trying line-out w/ analog attenuation.
So since these phones, on a properly amped system, with the right setup hardware and software, can reproduce incredible detail and throw a quite large soundstage around your head where individual instruments can be noted from one to the other. This is what they were designed to do, as they are built for the pro market. Not everyone is going to have the best setup, but as long as they have power, like most consoles, or mixing boards, Pro soundcards and interfaces, and DJ setups. These are a wonderful value in the audio world if you have the stuff to back them up with. A DJ is going to love them, as would someone needing something to knock around with in a studio, or an Audiophile that knows what they are doing and posesses the equipment that can drive these, not to mention the source materal (music) to play through them.
Sony liked themn so much they copied their design in their MDR-v600s, which are really bad phones, not to be confused with the V6s. The Shures offer a better earcup and headband material than Sony typically uses at this level, so the flaking of aging foam probably will not be an issue.
My friend. Too many people think it's all about the phones, and in a way your review is, but to be perfectly honest, at the leve, you were using them, we are reading a review of your associated equipment and not the phones. You did not have anything that could really push these the way they were meant to. This is why the XBs will sound so much better, they are forgiving and efficient, also not designed to show every weakness in your chain.
I hate when a good headphone gets a bad rap because it is thought it should work on an ipod or some integrated soundcard and media player. If you wish to call yourself an audiophile, you need more than just audiophile headphones! You obviously like music, but you need to research proper PC audio setups and then try them again when you have some basic associated equipment designed for the demands of a set of fine headphones like these. These are the real deal. They are durable, as the MDR-V600 (v700 only good phone in the consumer dj line for sony) won't break after abuse, the phones Sony copied the design from certainly won't.
Sorry to sound harsh, I just wanted to clear a few isconceptions I see always in PC Audio.
I suggest keeping the XB700s and getting a USB E-MU 0202. This is a DAC and headphone pre that can take on efficient phones very well, but it has a native ASIO driver, so you can begin learning how to get proper sound out of a PC.