Groomster, if you like the DT 770's bass response, it's going to be difficult to find a new high impedance can that can hang with it in terms of bass impact/quantity . There just arent a lot of high impedance cans being made any longer. You can always consider a higher impedance version of the same phone you have or maybe the DT 990 if you want to go to an open phone but still stick with a bassier phone. I have the DT 990 pro 250 ohm among a few others and they are may favorite phones. I like having a few different phones for different music genres or change of pace or mood or whatever the case. But, if I only could keep one of my present phones, it would be my DT 990's. The 990's also are strong in the bass dept. but I have never done a side by side comparison with the 770's.
AKG makes the 601 which is 120 ohm. Bass shy compared with the Beyer 990's and your 770's But Im liking them more and more with much of the blues I listen too, especially well recorded live blues (and even better with 5998 tube). Fischer Audio has the FA 011 at 160 ohm (hard to get in the states). MB Quart has some high Z cans but these are no longer made (See my thread "Hi-Z phone heads up thread" linking to a couple of ebay buy it now listings with 2 different Quart models on closeout). German Maestro is making high Z phones (took over production from MB Quart if think). KOSS has some High Z models and then of course there is the very, very popular Sennheiser. Im sure there are a few other manufacturers but that's all I can think of. And of course, there is the used market for out of production Hi-Z phones. Best you can do is check all the reviews that you can find and try to listen to some of the phones locally if possible.
Also, replacing the 6AS7 with a 5998 will lower the Cracks output impedance to about 70 ohms. That may or may not help your 80 ohm Beyers, sonically. But it is a very nice sounding tube (thanks again Jim) and you may or may not prefer it over the 6AS7. They are however, harder to find and signifigantly more expensive. And because of that, not a good tube to trust to any ole' ebay seller. One other thing about the 6AS7 ... NOS or good used, they can be had for relatively little money if you search for deals. Some of the USA JAN's or the RCA's come up all the time on ebay. You are kind of at the mercy of the sellers descriptions and their meters and ability to use them. But there are good deals to be had from sellers with good reputations. And sometimes a deal can be so good to be worth the small monetary risk. I picked up an excellent and what looks to be NOS RCA 6AS7 for, if memory servers, under 10 dollars. Also an excellent RCA JAN for around the same. It amazes me how many different size bottles they used for these tubes. I think I have 3 or 4 different size bottles all with, visually, the same internal construction. Personally, any differences in sound between these seem to be subtle at best when internal construction is the same regardless of whether they are labeled RCA, RCA JAN, JAN, GE JAN, CRC etc.. But YMMV. I think it is definitely worth picking one up when a good deal arises to compare with the sino that you have on the way. There is also the Russian 6H13C or 6N13. A lot on ebay, mostly from european sellers. Also several tube retailers seem to have these.
Im no expert on this stuff but hopefully some of this info is helpful.
Although, the headphone amp currently in the works is right up the same alley with a name like "Smack".
