With the E09K, you won't get a better DAC. So you could decide on a really good DAC/headphone amp pairing, and go with just the E17 for now as the DAC (there is cheap adapter to use the E17 line out). Or get them both now.
I thought the O2 was a pretty good upgrade over the E17 headphone amp. See the reviews in this shootout
The Schiit Magni is considered by many to be comparable to the O2. You could get it instead. Then get the ODAC or the Schiit Modi.
I have the Tube Magic D1 (not the Plus) in a setup in my house. I did compare it with the O2, but I would go with the O2/ODAC or the Modi/Magni over the TubeMagic unless you need the pre-amp capabilities or other connection options of the D1.