You might like the X1, they have a nice tad of bass and they are VERY easily eq-able. Oh my god what was I pleased listening to some bass heavy music with the fiio e10's bassboost on and the software eq on foobar to +10db. That literally made my ears tremble.
But eq isn't going to work with your iPod.

I used a sine wave generator with these, put it at 25Hz and cranked the knob on my Lyr... my head was vibrating. These cans go so low it's not even funny. Sure, they might not punch or rumble like Ultrasone 900s or any of Sony's XB series, but the bass is very quick, textured, and accurate. They definitely aren't basshead cans, they're more audiophile cans in my experience. I found the treble to be a bit hot and I experienced a lot of tinnitus/ear ringing during the time I had them (~3 weeks cumulatively -- I had the flawed 3rd revision and had to send them in for replacement) so I couldn't fully enjoy them. In comparison, my DT880/600s have no treble peaks like these do. I also find the mids to be much smoother and natural on the DT880s. If you have a decent EQ, software or hardware, you can make the HE400s rumble, but they will never be like that with their stock sound. Even with an amp like the Lyr, these will never satisfy bassheads, but those looking for an airy, accurate, textured sound will enjoy them. Unfortunately the treble peaks combined with the comfort issues (I found the velour pads to be very stiff and they left my head feeling strangely compressed) I had to return them. I'm glad to have experienced planar technology but it wasn't for me. Good luck in your quest.

If you have a lyr, you won't ever hear more powerful bass on the 400s

Thanks for the tip. I will look into the lyr. I'll probably leave HE-400 stationary if I pick up something like the lyr.

It's a great amp even with the stock GE tubes. If you get yourself some Amperex Orange Globes or Orange PQs it's even better. A bit more tube-y sound but it's so smooth it should be criminal. That's an extra $50 on top if you choose to go with them later. Personally I'd say stick with the stock tubes for a while and get a feel for them, then in a few months if you feel like changing it up, go to eBay and grab some.