[mgpyone]

I am looking to purchase a ER4 and people say that it needs a good amp to get it fully. Only amp will be enough but in the mean time, I am looking for a DAC too. 
Portable would be better like iBasso D-zero ? 
my budget is about 300$ .. 
I am looking forward to your suggestions. =)

 

[kmhaynes]

The ER-4PT does not need an amp as much as the ER-4S does.  The ER-4P has a reasonably low impedance of 27 ohms which makes it easier to drive and sound good with most audio players or computers.  
 
If you were buying the ER-4S, with 100 ohm impedance, you definitely would need a good amp to get the best out of it.
 
If you decide you do want a portable amp, I would look through the "Under $200 amp shootout" thread, looking for an amp that is "warm" sounding or has a bass boost, since the ER-4P does need some bass help with most music genres.

 

[mgpyone]

I see. I guess O2 amp or amp+dac combo would be a good choice. But I still want something portable =D

 

[ArmAndHammer]

Maybe look at the JDS Labs cMoyBB v2.03. It has bass boost, has plenty of power and a nice full, warm sound using the OPA2227. You can also OPAmp roll with this amp.