Belated reply but in the end I upgraded to HD580 phones, since they are going for very good prices brand new on eBay and the HD650 is still (over here at least) very expensive indeed (would rather spend the difference on cables that I still have to acquire and vinyl LPs).
Anyway the twin AD797s work extremely well with the HD580 as well - but if anything the combination are now possibly tad too smooth and polite. The HD580 (and the 600 / 650) are very smooth / lush / accurate relatively speaking compared to my older phones, so they don't necessarily need the smoothing qualities of the AD797 that were almost essential when using lower quality headphones that have a relatively brash character about them.
Neverthless, I have not really done much fine tuning yet. I still don't have the headphone cable I want (still using the stock one) and I'm probably going to get a decent one (much shorter too) from Larry at headphile, since I am a big fan of his work). I am also improving my turntable setup, since the turntable I had previously (Debut III) was just a toe-in-the-water exercise to see if I liked vinyl and was prepared to commit to it. As a front end, it has quite a lot of shortcomings that I only became aware of as I became much more experienced with vinyl. Now that I know I love vinyl, I've committed to a Project RPM5 with speedbox and a goldring 1022 cartridge.
All these things are of course going to improve the sound, with the ultimate fine tuning going to be with the op amps and cables. I suspect the new turntable / cartridge will actually smooth things out even more but will improve dynamics, timing and refinement (as well as reduce noise from the vinyl).
One thing I am looking forward to testing (have no idea if it will work or not) are twin NE5534 op amps. Reason being is that unlike most head fier's in this site's history, I was pretty impressed with classical music on a Creek OBH11. It certainly had a lot of shortcomings compared to the Cute for sure - such as overblown indistinct and muddy bass, a lack of space or "blackness" around the musicians and a shallower soundstage but it had a very warm, inviting, candid and engaging midrange / upper mid range quality about it. Some further research showed up that the NE5532 (used in the Creek) and the twin NE5534 are possibly under-rated under-appreciated, especially the NE5534. Other people have commented about those exact same qualities I was hearing in the Creek when using these op amps in other amplifiers.
If I could possibly get all the smooth and refined qualities of the twin AD797 mixed in with the warm, detailed and engaging midrange of (what I assume is) the NE5534 series, then this Cuye Beyond would be one very serious non-portable amp indeed for acoustic music.
I think it speaks volumes for the Cute Beyond that I have owned it for over 6 months and love it even more than when I first got it. I think the flexibility to tune it as other components in the setup have changed has a lot to do with it. Not just that, but the build quality too. You don't really appreciate how well built it is till you start comparing it to other components. Other components have come and gone as I go through the ubiqiutous upgrade paths, but one thing that I have never felt a dire need to change at this point is the amplifier. Maybe down the track sure, but for the moment I think i have my priorities right in terms of fixing up the source and getting the proper length and type of cables.
Once you get the two AD797 adapters, you can easily just buy 2 x OPA627 and try them out, since the hard part (adapter) has already been done for you. The AD797 will of course just pull off the adapter, since the adapter has it's own DIP sockets.
