While I've never had the privilege of owning the original HD-25's or the Momentum's I do own the Amperior's and can tell you that while they have quite a lot of bass it is more mid than sub (in other words you won't get the low rumbling in the 30-70Hz range like you would from say a pair of 80ohm DT 770's while listening to certain types of electronica) yet,a clean punchy and well controlled impactful bass around the 80-110Hz range. as far as comfort I have read more than a few reviews here and on other sites by people who own both the HD-25's and the Amperiors and one thing they all agree on is that both headphones have a very strong clamping force and thus a good seal,yet the Amperior's are more comfortable due to the ear cushions on them being more of a velour than pleather style fabric the HD-25's use, it is also agreed upon that the Amperiors are a step up in over all sound quality.
I wish I could compare all three headphones directly for you (as I would love to own all three ) and agree with bixby in that JerryBerry's thoughts on the subject are about as concise/succinct as you could ask for ,though I also understand your wanting to hear other thoughts on the matter from more users before financially committing to a pair of headphones you haven't had a chance to audition in person. I'd suggest checking out the reviews here as well as on other headphone sites .also it seems that there have been a lot of sales on the Amperior's lately (in many cases for less than half the MSRP) and so if you can pick up a pair used either here or on ebay or through a reputable seller in your own country I would highly suggest that you get them now before they end up costing as much if not more than their retail price (such as has happened with the Denon AH D2000's and other quality cans no longer in production). I was fortunate enough to purchase my Amperior's new in the box for (with tax and shipping included) only $152 which is nearly 60% off the retail price of $349,and I have been in love with them since they arrived and use them more than all my other headphones combined since they work great for so many genre's (everything from Electronica to Metal to Jazz to Punk Rock to New Wave to Classic Rock). it also doesn't hurt that they look amazing, are easy to drive through any portable player and that if you ever need replacement parts for them most are interchangeable with the HD-25.it would be hard to imagine a better sounding pair of headphones for under $200 (even if you get them used,so long as they are well taken care of).