Unfortunately I don't think there exists a headphone with the kind of bass response the JVC headphones had, at any price point. I have experience with the JVC HA-SZ2000 and a much cheaper headphone by the same brand (JVC) that came out a few years after the HA-SZ2000, called the JVC Esnsy SR-85S, which believe it or not had a bass response that was a near match to that of the HA-SZ2000, except without the damaging db levels. The massive 55mm drivers of the HA-SZ2000 shifted allot of air and truly did behave like a subwoofer. JVC achieved their intended goal with them, no doubt! It's a shame that JVC discontinued the HA-SZ2000/1000 and the Esnsy SR-85S, they where exceptional at what they could do, in my opinion. As I said, I doubt you will come across another headphone that had the low frequency capabilities of the HA-SZ2000, as far as I know, no other headphone on the market, since the release of the HA series, implemented the use of nano tubes and subwoofer technology into their design. The HA-SZ2000 was a true one off in the world of full size over ear headphones, it was basically like standmount/bookshelf speakers and a subwoofer all in the form of a headphone. A genius design. There is another headphone i've had experience with that I would say do come 3rd or 4th to the JVC's i've mentioned, for that subwoofer-like bass response, and that would be the Fostex TH-X00 Purpleheart (I had the 2015 non detachable cable version). I have never heard TH900's but from what i've read from reviews and comparisons by others who've owned/heard both, the TH-X00 Purpleheart had a similar, if not quite as powerful, bass response to the TH900's. If I may make a suggestion,- you mentioned HE400's (a headphone I have no experience with) but have you tried other planar magnetic headphones? I don't have a ton of experience with planar's, but from the one's i've owned/heard combined with my understanding of planar technology, planar magnetic headphones have an amazing ability to reach right down to the lowest depths of the frequency spectrum, well capable of emitting 16 Hertz killer bass response. They do typically sound linear, whereas dynamic driver headphones (such as the HA-SZ2000 and Fostex TH-X00/TH900) have a much more rounded bass response, which is obviously closer in nature to the kind of fat bass response subwoofers emit. If bass is your thing and you are looking for a similar sound to that of the JVC HA-SZ2000, I highly suggest/recommend you take a look at speakers. I have Mission LX-2 bookshelf speakers that have an immense bass response, whilst sounding clear and detailed in the midrange and high frequencies. The LX-2 might be within your budget, if they are available in your country.
