I've had a pair of SRH840's for about 2 years and I love how they sound, however they do get uncomfortable after long periods of listening and are lacking a bit in the bass department. I'm looking for a new pair of headphones under $400 that are closed back and I'd prefer to not have to buy an external amp/dac. So far the oppo pm-3's and the bose qc35 wireless have caught my eye, but I'd love some more recommendations. Thanks! 

You really owe it to yourself to get at least some sort of amp. If you are willing to shell out $400 on a nice pair of headphones, you really should due them justice by amping them. Even something as small as the Fiio A3, which is only $50, can make a huge difference in overall sound. Especially something like the A3 which has an integrated bass boost switch, since you said you'd like more bass than your Shure's. 
 
I know that wasn't a headphone suggestion, but I'm just trying to stress how much difference an amp will make. You should do yourself a favor and put aside $50 out of your budget for at least a Fiio A3. Anyone that is into music and enjoys nice equipment should amp their headphones.
 
As far as actual headphone suggestions go, the Oppo's are very good for a portable planar. Even those which are easy to drive, really open up when amped. Bass comes alive on them with an amp with bass boost. And since they are efficient, they won't need huge power to reach their full potential. 
 
Another suggestion would be the Fostex TH-X00. There is currently a drop open on Massdrop. FANTASTIC headphone for the $399, but would absolutely benefit from amping. IMO absolutely worth adding another $50 or so to your budget. Super comfortable, awesome low end, and solid highs that won't get annoying at high volume. One of the best "closed" cans on the market. I put closed in quotations since they are technically semi-open.

therealjnor said:

  I've had a pair of SRH840's for about 2 years and I love how they sound, however they do get uncomfortable after long periods of listening and are lacking a bit in the bass department. I'm looking for a new pair of headphones under $400 that are closed back and I'd prefer to not have to buy an external amp/dac. So far the Oppo pm-3's and the Bose QC35 wireless have caught my eye, but I'd love some more recommendations. Thanks! 

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What exactly do you plug your headphones into now?
PC, Mac, desktop, laptop, etc
 
For a closed headphone with good clarity, I prefer my Beyerdynamic T70.
Used, $229 + $15 shipping.
http://www.ebay.com/itm/112210730493?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

purpleangel said:

   
What exactly do you plug your headphones into now?
PC, Mac, desktop, laptop, etc
 
For a closed headphone with good clarity, I prefer my Beyerdynamic T70.
Used, $229 + $15 shipping.
http://www.ebay.com/itm/112210730493?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Click to expand...

I plug them into a focusrite 2i2 at the moment.