Got the same set and also like it a lot, I think it's very good value for money soundwise. Sometimes I could use a tad more warmth for the female vocals but otherwise great sound quality.

My Stax SR-507 really "sings" when listening to:   Allison Krauss - A Hundred Miles or More - Jacob's Dream Allison Krauss - A Hundred Miles or More - Down To The River To Pray Allison Krauss - A Hundred Miles or More - Whiskey Lullaby Anne Akiko Meyers - The Four Seasons - Concerto No. 4...

Would say it's significant enough to notice - more on some tracks than others of course. A bit less bass, a bit less monitor-like and more airy. Soundwise it will put you closer to the ER4S sound than without imho.

I picked up an 75 ohm adapter from fleabay like this one (not from that seller, just an example) when I bought my HF3 http://www.ebay.com/itm/Etymotic-ER4P-to-ER4S-3-5MM-plug-resistor-adaptor-/271363212964?pt=US_MP3_Player_Cables_Adapters&hash=item3f2e8206a4

Having only listened to my brothers RE-400 very shortly quite a while back I can't say much about them besides that my impression was positive.   I do however own the HF3 which basically is a HF5 with inline mic/remote and can say that they are excellent at noise isolation with the triflange...

AKG K3003i ≈ Heir Tzar 350 > Ety ER4 > Heir 4Ai > Ety HF3 > Klipsch S4   Have or had all

Listening to music just now with the chain Macbook Air > MF VLink II > Heed Obelisk DA > O2 > Heir Tzar 350. Sounds surprisingly well....actually awesome at 1x gain and with 15V psu.      

Am about to get an O2 and have the possibility to choose which two gain settings to get. Still don't find it being clear to me which will be optimal after reading the numerous posts in here. Thinking 1x gain for the first but dunno what second setting to get.   Use case will be either...

set to high

Tri flange usually gives me the best result. They go deep and cancels out any issues with different sound due to "optimal" placement in ear.

Quote: My 2Stepdance is dead silent until around 2 o'clock on the the volume with the iems I have tried which varies from Etymotic HF2 (low impedance), Etymotic ER4PT with 75ohm adapter (100 ohm) to Heir Tzar 350 (350 ohm). That said I never get that high on the volume so it's a non issue...

Regina Spektor, What We Saw From The Cheap Seats has some very good tracks Lady Gaga, Poker Face, Live at The Cherrytree House Session 

Had a short listen to an Argon HA2 a year ago or so and wasn't that impressed with the sound but we all perceive sound differently  Happy to hear that you found something you like the sound of - I do however think that you will find that the HD650 will scale well beyond the HA2 should you...

Quote:   Excellent value for money imo - annoying microphonics but good sound and the highs are just straight out amazing with female vocals. 

I have only had my HF3 4-5 months but haven't had any quality issues so far - use it 5-8 hours a week commuting to/from work.   No idea about fiio amps, never had one.   I would personally never use the ER4 with the ohm adaptor directly out of an i-device. Not that it can not play loud...

I have and use both HF3 and ER4PT (with 75ohm adapter - PT basically being a P). My personal experience is that if you compare HF3 and ER4PT that it's mainly the detail level in difference and HF3 perhaps being a bit more monitor like in the sound.   Making the ER4 an S is quite something...

ER4PT/S > Heir 4.Ai > Ety HF3 > Beyerdynamic mmx101ie > Klipsch s4i > Klipsch s4

Quote:   I have them both and will probably let the Heir 4.Ai go (got the 4.Ai with intend to replace my ER4) mainly to my preference in music (I like female vocals) in which ER4 is what you would probably call marginally but still quite noticable better, especially in the highs.  ...

Quote:   I find HF3 (HF5 with a mic/controller) very similar to the ER-4PT with 75 ohm adapter. HF3 has a bit more bass but don't feel they are warmer as such. HF3 sounds a slight bit more "monitor-like" and also lose a bit in the treble when directly compared to the ER-4. Also finding...

Pretty new to these forums aswell and surely no expert but have owned the s4i (which I wasn't that impressed by) and currently own the HF3.    When I looked for something with i-gear controls I found that it wasn't that easy to find iems with decent sound quality which also had inline...

Quote:   Sorry that I do not have same preferences in sound so can't recommend anything specific for you. Just want to say though that you shouldn't focus too much on whatever is recommended being new and flashy release date wise. I do for instance own the ER4 which more or less haven't...