I agree. I started with the STX and the HD650's. I was really enjoying this setup and then attended my first head-fi meetup. I then upgraded to a schiit bifrost (standard) and a valhalla 2. I first tried the valhalla 2 with the STX and immediately noticed how much more presence the whole setup had. Love the OTL designs with the 650 ( I have since listened to the BHC, Woo and a few others with the 650 and all are fantastic with their own little signature). Then I replaced the STX with the Bifrost and didn't notice much at the beginning. I do feel that after my ears and brain went through their burn in I did notice a difference.
Having said all that, I would definitely support your upgrade path; first the amp (a OTL if the 650's are your only HP, which i believe you stated they are) and then a multi-bit DAC. I am planning on sending my BF in for the multibit upgrade soon, if nothing more than to keep up with the new hotness, but also because I am running the original BF without the UBER, so I can skip all that nonsense and go straight to the MB revolution.
Good luck with your amp search!