you note that you arent rich but are looking at $250-300 headphone and $140 amps and $130 dacs. seems like a pretty hefty budget to me
while not rich, its still enough to look into the other reviews at head fi. there are models which are dac/amps for under $200 and arent all that bad.
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i'm running a pair of ath-m50s which are 38ohm currently and you are correct... you dont relaly need an amp for that level of impedence as the output from most devices is enough. not to say that having an amp doesnt give you a minor benefit (it does) but its not super pronounced. the 250ohm for the dt990 would require a decent amp though.
i use my m50swith my phone & ipod & laptop without a dac or amp and they sound fine. the only dac/amp i use them with is the receiver (connected to pc) for the rare times i use it while at home. honestly a dac is only needed if your source soundcard/output is bad or if you are an audiophile and expect the best. i thought about a dac in the past and so far i've not had to buy one. just a thought there. if you have decent sources already perhaps give them a try without a dac and see what you think.
if you intend to use this at your desk..
this was reviewed to be just a tad better than the e17
http/www.amazon.com/Aune-T1-Upgrade-Version-Amplifier/dp/B00A2QJMRY
cheaper link
http/www.aliexpress.com/item/FA035AA-AC-110V-BLACK-COLOR-Aune-T1-24Bit-96KHz-DAC-Hi-Fi-Preamp-Audio-Decoder-6N11/695478897.html
some other ideas mentioned here
http/www.head-fi.org/t/648456/best-dac-headphone-amp-at-150
i'm not saying that you cant go with a fiio just that there are other options out there which might be better which still fit in your budget. give it a look and roll through some head-fi posts on headphone dac/amps
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also of note: you said you like bass heavy. the ath-m50s are pretty decent in that regard. here is a sample clip comparing the m50x (the new version of the m50) with the dt770. it does have beats solo in there too (just ignore that part)
https/www.youtube.com/watch?v=4pK7QbLNjno