Is this a good setup/rig? advice please!

If you're not going to be using it as a DAC, I'd consider a better pure amp than the E17, such as an iBasso P4. To start with, maybe I'd hang on to the GR07s.  You haven't heard them until you've heard them properly amped. I use a P4 with my GR07s, with AD797s, the wideband buffers and dummy adapters on the two ground sockets (after much opamp rolling). This combination gives both good transparency and plenty of bass "slam".

I haven't heard the Cowons myself, but have read that they're good.

If you're going to use the Cowon as your main source, I'd make the DAC second priority, or get something like an iBasso D12 (about 75% of the P4's amplifier in quality terms, plus a very good hi-res capable DAC). I know they are both more expensive than the E17, but think of all the money you'll save by keeping the GR07s! I'm not even sure the Sennheisers are any better....

Just one thing - if your'e not using them already, try the Sony green hybrid tips. They offer a very good seal and are comfortable (and cheap!).

Both of the iBasso amps I have recommended offer the option of "rolling" opamps. This just means that the output stage amplifiers (which are in a single chip package, an "operational amplifier") can be exchanged to optimize the sound signature to individual tastes (a bit like rolling tubes). With these amplifiers, iBasso provides some opamps and buffers (additional amplification stages) for you to experiment with.

As for interconnects, the most important thing is to ue a LOD to bypass the Cowon's headphone amplifier stage (which is likely to be fairly grotty). Please bear in mind, though, that a Cowon LOD will be different from an Apple LOD....I'm not sure who makes dedicated Cowon LODs, though (if anyone). If there is no LOD available, you will need to use a short mini-to-mini cable (iBasso's are good quality, can't comment on the Fiio), and turn the headphone output up to about 75% or so. You will be double-amping, but you will have no choice.

By and large, as LODs are fairly short, any competently made cable will suffice (high purity copper plus decent connectors). Silver cable generally gives a slightly brighter sound signature. Don't spend too much on the LOD, though - much of this is just marketing!

I don't want to labour the point, but if you are amping and fitting the GR07s correctly, you will be amazed at what they can do. They have a particularly good soundstage, and will be comfortble with the right tips (and with the nozzles adjusted correctly). I am assuming you have given them the 100+ hours' burn-in. If not, they will sound sibilant and constricted - they really free up with use.

Although the GR07s are IEMs, they do need to be driven correctly. Either of the amps I suggested will do this (on the LOW gain setting!). Also, you can always move on to some good full-size cans later (HD600s, 325is, whatever you want - the P4 will drive them). If you're currently using your GR07s unamped and with stock tips, you haven't really heard them yet!

Also, a competently-designed, modern portable amp should not hiss. My P4 doesn't (not even when I'm using the Etys!).

The Sony hybrids are actually called "EP-EX10A" they come in a pack of various sizes, green being the middle one. I ordered mine from Amazon.

The gain is the amount of amplification applied to the signal. Generally speaking, the more gain, the higher the distortion, The lower setting on the P4 is unity gain (i.e. the same level as the input). For sensitive IEMs this is fine. Any more, and you will hear noise, distortion, and risk hearing damage if you accidentally knock the volume knob on-the-go!

Driving headphones is not about volume or "power", but about being able to control the transducer diaphragm correctly, and having sufficient headroom for dynamic peaks without clipping or compressing the signal. Damping factor is also important - think of this as being like engine braking on a car.....

Like driving a car, it's all about control, not just power.

As for system building, I would strongly advise against replacing everything in one go. Much better to focus on just one component, and to buy the best you can afford. This reduces the risk of rapid obsolescence! Once you really get into hi-fi, you will become extremely fussy.

So go for the Cowon by all means, but forget about DACs for now, keep your GR07s, and get a really good amp. The P4 is as good as many desktop amps at or near the price. Next, you should get yourself a pair of world-class fullsize cans (Sennheiser HD600, Grado 325i, Beyer DT880-600 Ohm according to taste - if you like one, you probably won't like the others!). Avoid the bigger AKGs with the P4, they really need a big solid-state desktop amp!