1. It could be a problem with your amp like you said.
2. My experience has been that full side headphones do not impress immediately in the same way that good IEMs do. Espeially since you are coming from listening primarily to IEMs it seems, its likely you haven't fully adjusted to the new presentation yet. I will say that I agree the SM3 bass is flatter and sometimes more accurate/tighter than HD650 bass; the only headphones of any kind Ive found that have better bass are Aurisonics. However ultimately if you pay attention, I think one thing you will find (once you ensure your amp is OK) is that the overall realism of presentation in the HD650 is on a completely different level than any IEM could physically accomplish; again the SM3 has a very unnatural/abnormal way of presenting sound and it may take your ears a while to adjust from that style of listening.
3. The mids on the HD650 sound veiled to some; same goes for SM3, but the HD650 is a full sized open headphone which means its vocal presentation is much less direct and forward than any IEM, especially the SM3, which literally puts the vocals inside your head and almost makes you feel like you're the one singing.
4. The HD650 uses big huge dynamic drivers, which are at least as detailed as the UM3X and SM3 when properly amped, but will never be as fast as mutli-BA setups.
Hope this helps
Ultimately they are very different headphones, so no matter how similar the sound signatures are, never expect them to be the same. I made a similar mistake projecting expectations of a full-sized PX100-ii, and was similarly disappointed at my very first listen, but quickly came to see that they are simply very, very different.
2. My experience has been that full side headphones do not impress immediately in the same way that good IEMs do. Espeially since you are coming from listening primarily to IEMs it seems, its likely you haven't fully adjusted to the new presentation yet. I will say that I agree the SM3 bass is flatter and sometimes more accurate/tighter than HD650 bass; the only headphones of any kind Ive found that have better bass are Aurisonics. However ultimately if you pay attention, I think one thing you will find (once you ensure your amp is OK) is that the overall realism of presentation in the HD650 is on a completely different level than any IEM could physically accomplish; again the SM3 has a very unnatural/abnormal way of presenting sound and it may take your ears a while to adjust from that style of listening.
3. The mids on the HD650 sound veiled to some; same goes for SM3, but the HD650 is a full sized open headphone which means its vocal presentation is much less direct and forward than any IEM, especially the SM3, which literally puts the vocals inside your head and almost makes you feel like you're the one singing.
4. The HD650 uses big huge dynamic drivers, which are at least as detailed as the UM3X and SM3 when properly amped, but will never be as fast as mutli-BA setups.
Hope this helps

Hello,
I just recently received a pair of HD650s and I am very worried/disappointed. I'm hoping you owners can help me out.
My chain is the following:
16bit 44.1 .flac > Schiit Modi (DAC) > Bottlehead Crack + Speedball Upgrade (amp) > Sennheiser HD650.
SUMMARY:
1. The treble just doesn't sound good. It's just very unnatural and artificial. It just doesn't sound believable at all. Hearing treble on my old Westone UM3Xs was better.
2. The bass presentation was very, very disappointing. The bass is not dynamic at all; it just sounds like a big floppy woosh that hits without any sort of character. It's like having someone with a big drum who hits it exactly the same every single time. The bass is not tight nor punchy.
3. The mids aren't particularly impressive either. I find the mids on my Earsonics SM3s sound better... I really thought that big cans would blow my universal IEMs out of the water, but so far, no. I mean, maybe it's not the signature of the HD650s to have very forward mids, but the voices don't sound lush or anything either (even at a greater distance than something very mid forward like the SE535s or SM3s).
4. The best thing I can say about them is that they seem very flat from top to bottom, and are fairly revealing (I can instantly hear a bad recording and exactly what is a cheap sample vs. a good quality recording). Electronic music often sounds bad on them because of the way it picks apart the often crappy synths and samples some producers use.
5. Overall, I just don't find anything they do very realistic or natural. It's just very unnatural sounding. Voices sound like they're being played to me (I'm used to vocals that sound like they're realistically singing to me). Music just sounds like it's being directed into my ear, rather than any kind of immersion. The treble is perhaps the worst part though; it just sounds fake.
I am very let down by the HD650s. I have been asking a lot of questions on the Bottlehead forums about the amp and how to make sure that I'm getting optimal amp performance. I'm doing some resoldering in the amp tomorrow on the capacitors to make sure everything is proper. I even ordered a more expensive and highly reputed tube for my amp (a Tung Sol 5998) to try and warm up the sound and to make the most of the headphones through the amp, so I'll have to see the difference when that arrives. In the meantime, however, I would like some feedback on the headphones themselves from good owners like you guys.
There haven't been a lot of burn-in yet on the headphones. I've been running pink noise at higher than listening volume into them, but I've only gotten to put in about 5-7 hours so far. Should I expect big improvements at 50-100 hrs?
I have the silver driver HD650s!
Thanks,
Pepper
P.S.: They're still better than your average headphone by a lot, but their sound isn't very alluring. Do you think it's maybe just a sound signature problem? I personally like warmth, lushness and very good vocal rendition (mid forwardness).