I decided to buy the M Stage in the hope it would adequately drive my k701, on the basis "if it doesn't work out, if havent spent much money." However I found the results to be quite interesting. I thought I would say what I found with my different phones, as it appears a partner for the M Stage should be chosen with care.
For reference, my other amp is the superb Ben Duncan Phones 01 and the sources tested were a Rega Planet, Rega Apollo and Rio Karma DAP. I have however, tried many, many amps in search of something that works with the k701
A match made in heaven! For this first time I can understand why people rave about the K701. They just seemed to come alive, gone was the hollow, shrill and bass-light sound I got from other amps. Instead we have well ballanced and controlled bass, beautiful mids and highs, and acceptably fast timing. Honestly, "bought to life" is the best phrase I can think of to describe the M-Stage's affect on the K701. I have not heard the K701 sound as good with anything cheaper than £800 (the amp in question was a Chord Toucan, and even that sounded average with some other phones). I found my self re-listening to favourite CDs, just to experience the new insight this set-up gave.
Negatives? The highs did not have the ultimate extension that, say, the Phones 01 and some of the more expensive competitors demonstrated. Also, accoustic instruments could sound a bit "plasticky" and lacking in realism. I don't know if this is the character of the K701, or perhaps the M-Stage is not accurately reproducing some of the very high harmonic frequencies, or both.
A match made in hell! As the K701 is supposed to be harder to drive than the HD600, this was a real surprise. I found the sound to be almost muffled; the infamous Sennheiser veil was there in spades. Bass lost the impact these phones demonstrate with well-matched amplification, although extension was good. Really, they just sounded like a dreadful, sluggish, muddy mess and I couldn't listen to them for long! I must add these phones can sound sensational, for example the Phones 01 is a great match.
ER4P and ER4S (with converter)
I have never heard so much bass from the Etys. In ER4P guise the bass was actually overpowering! I could not believe the Etys had the ability to reproduce so much bass, both in extension and perceived volume. However, the M-Stage did not do justice to the ER4's truly impressive capacity for treble extension, and as a result a lot of music was somewhat lifeless. I noticed this aspect of the M-Stage with the K701, but to a lesser extent. Given the similarities in presentation between the ER4 and K701, I was surprised at how much more they were affected by the M-Stage's character.
I actually think the M-Stage would be a good back-up amp if you have ER4P/S, but occasionally crave an eardrum-loosening bass session!
UE Triple.fi 10
If the ER4 was bass-heavy, you can guess what happened here. The bass pretty much blotted out the rest of the music, but did not sound particularly controlled (although we're still talking about a BA earphone here). The presentation was also very coloured and had the "muddiness" I noted with the HD600.
If you have the K701 and want a budget amp, don't hesitate. I have heard nothing that sounds this good, even with some amps 6x the price. It's not perfect, but can make for a great listen and I have seldom felt so connected to some of my favorite tracks. However, if you're not an AKG owner and don't plan to be, try before you buy to avoid disappointment. The M-Stage can sound great, but only with the right phones.
The question is does the M-Stage/K701 combo work so well because:
a) It's a great amp driving a great pair of phones
b) The M-Stage has overblown bass and rolled-off highs. The K701 can be bass-light and too shrill. Together they counteract eachothers weaknesses to somehow produce a listenable sound.
To be honest I don't know. Be it combined technical excellence or freak of nature, I'm happy with the results!