So got the onkyo few hours ago
Here are some comparisons and impressions between the modded dolphin and fc300
The dolphin's have some bass boost toward the sub bass and it is quite good, tight controlled and little overall bleed into the mids. I like my headphones bassy but don't mind if they aren't especially bassy/basshead headphones. The onkyo have quite a bit more emphasis towards the mid bass and I would call these some standard basshead headphones. It is rather tight and controlled as well and has overall more bass quantity than the dolphins. Though as a result of having more bass there is some bleed into the lower mids. Overall with bass I wouldn't say one is too much better than the other as they both focus on different things, the dolphins with sub bass and the onkyo with mid bass but certainly more. If I did have to lean one way I would say the dolphins for less bleed into the mids and slightly better control though that may be a result of having less bass quantity.
Both mids are quite fantastic both quite rich and fairly detailed on both. The onkyo as I mentioned I found had some bleed from the bass. Overall both are very competent in the mids, male and female vocals sound quite good. No real victors for me in the mids, maybe the dolphins.
In the treble there are some notable differences. As a result of the foam mod with the dolphins the treble became a bit more forward and while I don't mind that so much I did notice a bit more sibilance as a result. The dolphins overall became a brighter headphones and while I am quite used to it and can eq it down if a song is especially sibilant that may not be for everyone. The Onkyo on the other hand doesn't have an as forward treble but it is forward enough to really sound quite nice and detailed without sibilance issues. I have another pair of headphones that are quite bass boosted with a quite recessed treble, the phiaton ms 400, and those have a quite dark signature. The onkyo in comparison to the phiatons aren't a really dark headphone but aren't a really bright headphone so it strikes nicely in the middle. If I would have to say I prefer the treble of any I would say onkyo as they aren't as laid back to give a dark sound signature but isn't as forward to hit sinilance problems.
Had I not removed the foam from the dolphins this might of been a closer contest, but as I have this hands down goes to the dolphins. Soundstage after the mod has really improved and I have fallen in love with this headphone again. Even with my issues with the sibilance as a result of the mod I would handily do it again if I had to (though I wouldn't want to since that was a pain in the rear). The soundstage on the onkyo aren't especially small it is quite decent and nice. I just really would say the dolphins take soundstage quite handily.
The dolphins is over ear though my ears touch the inner cup slightly. I have used the dolphins at work without the foam mod and it gave my ears even less space and after a few hours my ears did end up a bit sore. The onkyo on the other hand is on ear though for an on ear ear the cup size is quite massive and isn't uncomfortable for the short time I've used them. My earlobe is touching all of the inner cup with the exception of the top and bottom of my ear lobes which touch the pads. I would imagine these would get uncomfortable quicker than the dolphins but I have only had them for a few hours and been taking them off and on a bit. So I would say overall comfort I would say go for the dolphins though I was moving some boxes outside in this dumb and hot California heat earlier with the onkyos and it wasn't too sweaty for me.
Both are quite good, companies did not cheap out on real crappy pads and though each are not real leather the pleather used works quite well. When I had the onkyo on earlier I missed a few calls from my mother and she was not too pleased about that. At work with the dolphins my boss has gotten upset a few times as he tries to get my attention while I had the dolphins on and I couldn't hear him. Overall I haven't really reached a conclusion between the two as I have only had the onkyo for a short time.
So the onkyo came with the headphones, some reading material, a plasticish bad and one cable. The dolphins came with two cables, a hard shell case, some reading material, and the headphones.
The dolphins have some nice metal parts and a lot of plastic materials. The onkyo is mostly plastic though really hard plastic and some metal parts here and there. It swivels really similar to a lot of other headphones. Both are built quite nice so no real conclusion on my part and really based on preference. The dolphins did have a bit of squeak in the left cup but I fixed it with some wd40.
Both are really evenly matched headphones, I would say in my opinion I like the dolphins more for it's bigger soundstage. The onkyo I would recommend for bassheads with a real good overall sound and real nice treble. The onkyo I got off amazon for $108 if you factor in tax but I am a prime member so I went for two day free shipping. The dolphins I got secondhand from bhazard and price to performance I would say the dolphins perform better with the mod though it does give a more forward treble and give some sibilance. The onkyo are a really great value and I would say they handily outclass my phiaton ms400 in nearly every category soundwise for my bassy headphone.
These are only my brief impressions of the onkyo and even without any form of burn in I did find the onkyo to sound quite good. Not a too big believer in burn in though.
tl;dr: dolphin sub bass based, foam mod removal = more soundstage, bit more sibilant, over ear
onkyo mid bass based, much bassy, beauty treble, on ear