I'm writing this because I feel the need to defend this little giant-killer.
There are a lot of negative reviews, and while I am not on the "Schiit-wagon" at all, or a fanboy of their products, I now have a better idea of what kind of amps are out there, after owning this for two years and having upgraded dramatically to the Lehmann Rhinelander (around $500) and then the Lehmann BCL Linear (around $1200 or more). Guys who have been into hifi for years are shocked when they hear I have a headphone amp that costs $100 and basically reflects the source with no extra frills.
First of all, there is a sense in which no, I would not recommend this amp for IEMs, particularly super low-impedance Shures (I have the 425 which has 18 Ohms!). They have to be listened to very loud before the channels are balanced. BUT, and this is a big BUT, most solid-state headphone amps without a gain switch are going to give you the same problem of low volume channel imbalance (including the Lehmann Rhinelander which is 5X the price), and it is still probably better than a headphone output on a CD player or home receiver, so it may be worth it for that. Otherwise, if you listen to iPods or iPhones or better DAP sources you are probably not going to gain much through your IEMs. (This problem may be less dramatic with the new Schiit Magni 2, which I have not tried yet, but is reported to have a gain switch.)
Efficient headphones like Grados likewise, don't seem to benefit that much, but the Magni does add significant sub-bass as long as the extra high-range energy doesn't bother you.
I do not have the accompanying Modi and have generally used HRT products for my USB output sources. Unfortunately, from my Lenovo laptop, I often got a very high noise floor when listening to Beyerdynamic phones, which are notoriously sensitive to that.
With dynamic phones that have over say 70 Ohms, and from what I've read, efficient Orthos, the extra power does only good things for headphones over a regular headphone jack. Assuming you have a decent source and cables, such as a non-portable CD player or relatively good DAC, you are going to find the Magni a fantastic value. Even if it's just a stop along the path of searching for a better amp, I think this is a good value as a very punchy, fast, dynamic, and powerful, headphone amp.
Every amp has a sound, and this is probably just a bit on the V-shaped side of things, with slightly cooler mids and warmer lows and highs. This is well-known by now, which is why I had to get rid of my Beyer 990 Pros, but on the other hand, my more balanced sounding Beyer DT 150s sound fine.
Now that I have tried better amps, I realize that power filtering is really vital for sensitive headphones, and for some reason the Magni does not seem to have a very good soundstage, but this could be less of a problem with headphones that are not really oriented towards that direction.
After years of IEMs and dynamic headphones, I think I'm going to finally go for some Orthos, and I'm definitely keeping my Magni. I highly suspect that with a decent CD player (even an older used Marantz, Sony, or Denon, Phillips, etc.) or a record player with some sort of clean phono stage and the Magni, plus any number of mid-range impedance dynamic phones (80-300 Ohms) that are relatively dark in signature, one of the many efficient Orthos like the Hifiman line or a modded Fostex, I'm going to be very happy (even though I already have a much better amp).
With the Magni you can find a good used source and spend your money on getting the best over-ear headphones you can afford. Then, as you save, upgrade to a better amp, or not. (I do not, however, recommend the Magni with a cheap USB powered DAC that will leak in USB power noise!)
Put it this way: if you want to listen to MP3s all day, just continue with your DAP and IEMs or one of the zillion types of decent efficient headphones out there that don't really need an amp. But if you want to enter the world of hifi music, you will need to upgrade to a 'real' source at the least, and these may not have headphone jacks. So, here's where the Magni comes in handy at the right price.
There are a lot of negative reviews, and while I am not on the "Schiit-wagon" at all, or a fanboy of their products, I now have a better idea of what kind of amps are out there, after owning this for two years and having upgraded dramatically to the Lehmann Rhinelander (around $500) and then the Lehmann BCL Linear (around $1200 or more). Guys who have been into hifi for years are shocked when they hear I have a headphone amp that costs $100 and basically reflects the source with no extra frills.
First of all, there is a sense in which no, I would not recommend this amp for IEMs, particularly super low-impedance Shures (I have the 425 which has 18 Ohms!). They have to be listened to very loud before the channels are balanced. BUT, and this is a big BUT, most solid-state headphone amps without a gain switch are going to give you the same problem of low volume channel imbalance (including the Lehmann Rhinelander which is 5X the price), and it is still probably better than a headphone output on a CD player or home receiver, so it may be worth it for that. Otherwise, if you listen to iPods or iPhones or better DAP sources you are probably not going to gain much through your IEMs. (This problem may be less dramatic with the new Schiit Magni 2, which I have not tried yet, but is reported to have a gain switch.)
Efficient headphones like Grados likewise, don't seem to benefit that much, but the Magni does add significant sub-bass as long as the extra high-range energy doesn't bother you.
I do not have the accompanying Modi and have generally used HRT products for my USB output sources. Unfortunately, from my Lenovo laptop, I often got a very high noise floor when listening to Beyerdynamic phones, which are notoriously sensitive to that.
With dynamic phones that have over say 70 Ohms, and from what I've read, efficient Orthos, the extra power does only good things for headphones over a regular headphone jack. Assuming you have a decent source and cables, such as a non-portable CD player or relatively good DAC, you are going to find the Magni a fantastic value. Even if it's just a stop along the path of searching for a better amp, I think this is a good value as a very punchy, fast, dynamic, and powerful, headphone amp.
Every amp has a sound, and this is probably just a bit on the V-shaped side of things, with slightly cooler mids and warmer lows and highs. This is well-known by now, which is why I had to get rid of my Beyer 990 Pros, but on the other hand, my more balanced sounding Beyer DT 150s sound fine.
Now that I have tried better amps, I realize that power filtering is really vital for sensitive headphones, and for some reason the Magni does not seem to have a very good soundstage, but this could be less of a problem with headphones that are not really oriented towards that direction.
After years of IEMs and dynamic headphones, I think I'm going to finally go for some Orthos, and I'm definitely keeping my Magni. I highly suspect that with a decent CD player (even an older used Marantz, Sony, or Denon, Phillips, etc.) or a record player with some sort of clean phono stage and the Magni, plus any number of mid-range impedance dynamic phones (80-300 Ohms) that are relatively dark in signature, one of the many efficient Orthos like the Hifiman line or a modded Fostex, I'm going to be very happy (even though I already have a much better amp).
With the Magni you can find a good used source and spend your money on getting the best over-ear headphones you can afford. Then, as you save, upgrade to a better amp, or not. (I do not, however, recommend the Magni with a cheap USB powered DAC that will leak in USB power noise!)
Put it this way: if you want to listen to MP3s all day, just continue with your DAP and IEMs or one of the zillion types of decent efficient headphones out there that don't really need an amp. But if you want to enter the world of hifi music, you will need to upgrade to a 'real' source at the least, and these may not have headphone jacks. So, here's where the Magni comes in handy at the right price.
Like almost every Class AB SS amp, it tends to be a bit cold sounding for my liking, over the long term. That's why I gave it a 4 star rating, instead of 5. But it is definitely a very competent amp, and is so good for the money as to be almost a gift.