@goodyfresh vs FiiO BTR5? Ok, here (FiiO as USB DAC/AMP; both from PC):
- AK PEE51 has a cleaner sound with noticable darker background and protection against EMI/RFI is better (much less hiss when using near smartphone working on 4G);
- the sound of PEE51 has better control, is more natural and with higher level of transparency; BTR5 sound is more digital, with more obvious coloration;
- PEE51 has a more textured lows, denser, yet well controlled, with better extension into subbass; microdynamic is better with AK, but overall level of macrodynamic depends on paired IEMs;
- the mids of PEE51 are smoother, with more focus on better rendered vocals; BTR5 has clearer, sharper mids with touch better separation within "X" axis;
- the highs are a bit boosted with BTR5 that cause it to sound way less natural; the level of details is nearly the same, but FiiO has a touch more resolution;
- the soundstage width is better with FiiO, PEE51 has better height, depth is similiar (not very impressive on both); stage with AK has more 3D-feel and PEE51 work better with imaging and showing correct position of instruments.
FiiO BTR5 is a good product, but I don't find it to be spectacular or something worth flexing. Maybe with darker sounding IEMs it could be good option, but otherwise anyday I'm choosing PEE51. The best for you is to compare both and choose one you like better.