Hope the members can help me out on this and any advice is really appreciated.
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big jump going from on ear to over ear and closed to a open headphone as well as portability vs non portable. Once the new he400 comes out I'm sure prices will drop even more on used old versions but your going to need a amp to go with them. The he300 I believe is their lightest headphone so it should help the comfort but hifiman products are not very comfortable. Regarding the ef3 a few Chinese websites said the ef5 replaced it and ef3 was never released dated from 2011. It looks like a ef2a but without the dac. Not sure how it compares to the budget friendly magni and vali.
I think the lowest iv seen the he300 was $170 so a $80 amp on the ef3. without shipping charges.
You should go see if any stores around you have a open headphone to try. Sennheiser has alot of lower end open headphones you might find at best buy. Just to demo the open air part.
If it was me though id check the used section for hifiman products in near mint condition and probably a magni as my first desktop rig.