There are many discussions on Orchid, but there are only a few on Istanbul. Therefore I want to post my experience when comparing them.
First and foremost, they are more alike than different. They are from the same family, using almost the same design and components. You won't be surprive when they have the same MHDT signature sound. You cannot go wrong from either of them.
I am a fan of MHDT, I had Havana back in the day, and also had balanced Havana. One can say that Havana made MHDT famous. Istanbul is the sutble evolution of Havana. Havana --> Stockholm --> Stockholm 2 --> Istanbul. Every time, it improved a little bit over the previous generation. Why do I like MHDT signature sound? It is hard to explain. If I have to explain in words, I would say silky smooth, less digital, and it conveys the romantic of music, similar to the experience of vinyl. In addition, it plays ALL music well. I mean all, even not audiophile standard music. I had tried some hi-end DACs, they are very 'accurate', and good for high standard or audiophile records. But I can't bear it with so-so recorded music. In the end, I don't want to limit myself to listen to only a few audiophile records.
Back to Istanbul and Orchild comparison, I have not emphasize enough they are more similar than different. Actually I wonder, the differences of changes tubes may have more effects than the differences between these two DACs. I would say Orchild has a little bit more details, and a bit more natural, but Istanbul is a bit more musical and romantic. I like both. But if I have to pick, I compared them in two different systems, and I like each one for different reasons.
In my bedroom system, I sit very close to my speakers (Cabasse Feralla 401), 3 ft away without peramp, I like Orchild more. It has more details, and I like it more, because it presents music more natural and real in the very close distance listening.
In my main system, I sit 24 ft away from my speakers (KEF Reference 203) with Freya+ preamp. I like Istanbul more. Because of sitting far away from the speakers and no acoustic treatment in the room, I don't hear the edge of details from Orchild. While I feel more of the music from Istanbul, it presents music more musically and romantically in that setting.
For the reference, I had the perfect score in measurement RME-ADI-2 DAC. It is a very good modern DAC. It can do anything, and did many things quite well. If you have it with PC, upsample all the files to DSD and play, it sounds very good. But I sold it, because a) I don't use many of its features, b) even it is accurate, but I just enjoy music more with MHDT.