I don't know Calyx, since their desktop products are top notch but as making a dap, it will be their first shot, I keep my hopes relatively low. Soundwise it might be good but real life user experience can be pretty bad. As for X7, I already skipped X3 and waited for X5, I don't think I can wait more than a year to get X7. Also, my SE5way cries to get a better source, I must retire Clip+ to use in the gym only (btw it can't drive my gym headphones KSC75 properly either ).
Your mod idea seems interesting but since I am the worst DIY person in the world, I rather not to touch any device like that, I must skip it. I can say, of course 627 can add nice things on top of X5 but there is a big chance that X5 alone might be very satisfying already. I am also considering bringing things to the other levels but instead of going portable, I might look into transportable setups. If there is no chance to connect M8, I am seriously considering Wagnus Epsilon S amp to pair with it. It won't be a daily rig but when I am willing to carry an extra, small shoulder bag, I can take 2 devices with me or when I am traveling (new home, hotels etc).
Budget-wise, well we are kind of in the same boat, once again. As an abroad master student, I am also having difficulties of saving money for audio gear (God knows how I suffered to get SE5way). So while X5 keeps everything either perfect or great with no real big con, at that price I think, I or we can't find a better deal.